So let's begin. Yoshimitsu. Viewed in Tekken 6 as being one of the worst characters in the game. Why the hell would anyone want to learn him? I like to think you need a combination of a little bit of crazyness, and a hell of a lot of intelligence to be effective. You have to be a bit crazy to sometimes disregard what frame data tells you, and be willing to throw out risk with the intent on confusing your opponent. To elaborate on that, I need to go back to the beginning.
Since DR, I've tended to use more powerful players. My mains were Heihachi and Bryan, probably because I could be a bit brainless with the offensive and mash buttons without really understanding what I was doing. Then 6 released, and I picked up Lars. Great, an even EASIER character to win with! But eventually I hit a wall and felt like I learned all I could with the character. Then I wanted to help a local player improve his gameplay with Yoshimitsu, so I can't teach unless I learn him myself right? I learned the staple combos and learned the top 10 moves an abused sword sweep like most players. I kinda had fun with the character and I started to use his stances to act goofy mid fight. But that's all it was, acting goofy. I eventually hit a wall and couldn't progress for a good amount of time, and contemplated dropping the character because I was getting tired of losing myself. Then I started to browse TZ, and was fortunate enough to be taken under Noodlehead's wing, and he started to introduce me to mindgames and gimmicks. I was learning something new everyday. The notion of learning in itself is pleasing to any player, and that is what gave me the initial drive to continue to play him. Noodlehead's style was unlike any other I had seen at the time, it was goofy, stance heavy at times, and effective. I couldn't really touch him and I realized I had a lot to learn. Thereafter, the drive to learn and improve is what kept me going while receiving great training.
Over a period of about 6 months, I was doing nothing but losing. I was learning each day, but it was so much information to take in that I either couldn't remember it, or i couldn't apply it properly as I would confuse myself and just freeze at times. I started to believe that if I was able to confuse myself, I would most certainly be able to confuse a non-yoshi player. Then one day things just clicked: I was fluid with my movement, and was able to switch between stances without frying my own mind in the process. My ideas were finally clicking with muscle memory, and I started to win on occassion. Then I was getting more comfortable and started to win even more, and people started sending hate-mail saying they hated random yoshis and such. That playstyle wasn't random... it was just one or two setups I had learned, combined with some of my stance-heavy playstyle. I wanted to learn more and I'm still learning something new all the time. It's so much to take in, which is why I feel you need to be very intelligent to be effective.
Right, so enough of that noise, let's begin with my movement style, something I've dubbed:
Manji Health Dance
During a match while using yoshi, I almost rarely BDC. I find that taking advantage of his MED and IND have far greater advantages to forcing whiffs at most distances. Many players refer to this style as annoying and random, but they don't realize I'm always looking to set something up. Ideally, when you're doing any motion, you should have a desired outcome for what type of attack the opponent is going to use. You should always have an intent with your inputs, if you're random you might get away with it for a bit, but ultimately the outcome won't be pleasant. Keep that in mind if you decide to use any of these traps and theories. Bear with me for my range descriptions... I'm not too sure if this is even accurate to how I use it, but we'll see. I basically judged it by using range 0 as right up, range 1 is one bd, range 2 is 2 bd, etc.
At range 0 through range 1.5, this is where I do my BDCing. Stances are obviously not safe to attempt this close.
Range 2 through range 3, this is where I like to use MED stance. Many players feel like this is free hits for them once they see your back turned around here, but yoshis evasion is key. MED~b is ok to a point, sometimes I'll throw this out either right after entering my 'personal' MED range to see how the opponent reacts (either going back into MED, or punishing given the opportunity), or to gain a slight bit of distance if I'm going for a BT flea.
Where yoshi really shines is using proper MED~b~db. This creates a wide range between the opponent and once yoshi starts the evasion, it's guaranteed (as far as I've experienced) that yoshi will evade any attack. The ballsy players will throw out a launcher, this giving you plenty of time for a free ff+4, or CD1. Occassionally I like to use this at even farther ranges for MED3, which gives a free u, n, 3 for a total of 43 damage. Or if you're execution is on point, MED3~ BT Flea for shenanigans and a very annoying unblockable with a very large hitbox.
Range 4.5ish and on is where I feel comfortable doing IND attacks. This is also where having lots of knowledge of yoshi's BT stance also pays off to take even more advantage of the distance. IND while facing forward I typically play with IND 3, 3+4 mostly by gauging the opponent distance and just using proper cadence. Conversely BT IND3 is insane for evasion.
The whole purpose to doing the dance is not only to frustrate the opponent, but to spend as much time possible to recover health, while still remaining relatively safe, or possibly be in a position to create whiffs. Moving on.
I feel like this is something that is greatly overlooked by a good majority of players. When players enter BT two things typically happen: the player in BT is in panic mode because they can't block, and the opponent gets a lil excited because... well... his opponent can't block. This entices them to be overly aggressive and more inclined to make mistakes. You can take advantage of this with yoshis pretty solid BT attacks:
Generics: BT 4_d2_d4_uf4_grabs
Yoshi specific: BT d1_db2s_2_1,2_db1+2~ 3_4
The generics are pretty self explanatory. But bear in mind that a BT grab is a pure guessing game, so use that to your advantage.
BT db2s are great for creating some quick space and can easily be followed by BT d1 since it has some good range. If your execution. BT 1,2 is something I rarely use myself, but sometimes you're able to fish a CH yielding a combo. BT 2 is good if you want to set up a flash attempt or maybe even CH fish. One thing I feel that is overlooked is BT bad stomach. Bad stomach 3_4 alone is good for creating space, but when it's backwards it's pretty nasty. Good damage, safe on block (you can sometimes bait a BT uf4 after being blocked depending on if the opponent tries to punish), but also juggles on CH, all while covering a pretty big distance.
Fighting Without The Sword
Probably one of the most overlooked stances yoshi has to offer. I'm in agreement with Noodlehead in that this is more of a defensive oriented stance. Yoshi gets a pretty decent 10f punisher that wall spats (2,1), he can finally use a generic crouch punch (db1), he gets his old df2 back, and above all, flash becomes even more insane. You get a FC version, more range, though slightly slower. The main drawback to NSS is you lose DGF and Flea stance, as well as a few unblockables. But again, you should be playing more on the defensive side using this stance. Yoshi will also get some more flash setups, but more on that later. Anywho, below are a few of my go-to moves that are NSS specific:
1,2,1 -- for some reason, people tend to get hit by the 3rd hit for some minor damage.
2,1 -- 10f I think, so-so damage, sends the opponent far, and wall spats.
kin 1 -- highly underrated. Quick, hits mid and grounded, forces crouch, safe, and on hit it puts the opp FDHT.
FC Flash -- a high crush flash that leads to a full combo? Insane, make use of it.
uf 1+2 -- seemingly useless. This is the swordless version of the death copter, but I've found that most players haven't even seen this move and just kind of freeze when they see yoshi do the empty hop. What separates this from a generic UF, is that when yoshi lands, he is in a regular standing position as oppose to FC. I like to use this to mindfuck the opponent while buffering a qcb1+2 grab.
Be sure to incorporate not only these attacks, but yoshis regular stances and standard 1ss attacks for maximum effectiveness. NSS is used so seldomly that many opponents don't know how to react to it.
Hands down yoshis best move, and the fastest attack in the game. It takes some time to understand the range and it's benefits without being the sort of player that just throws it out randomly hoping for the best. What most people don't know, is there's 5 variations of flash:
1+4 - standard. 6f, shortest range.
4~1 - 7f, slightly longer range.
NSS 1+4 - 8f, even more range and gives a f+1+2 B! combo
NSS 4~1 - 9f, even more range and still gives f+1+2 B!
NSS FC 1+2 - 10f? unsure of range in relation to regular NSS, but it's a high crush and still gives a full combo.
This move can safe you from a LOT of headaches and take away a lot of players mixups, especially if they're the sort that likes to barrage you with quick jabs and pokes (Nina, anyone?). I'll lightly touch on the subjects just to give you an idea of where to begin:
Taking away the mixups (bear with me, I don't know either of these characters, credit goes to Inca as the bulk of this paragraph is loosely plagurized from his bible): Ever been frustrated with bruce's CD motions? Not sure if he's gonna go for that annoying low kick or a mid launcher? Flash him. Works perfectly after that left uppercut~ cd motion and will stop any of his options. Same for Leo, whenever he enters the BOK stance, flash will interrupt all his options surprisingly. To go even further on taking away the mixups: NSS can be a life saver vs Julia. NSS flash will stop ALL of her spin options, as well as punish her uf3,4 giving you 70+ damage for something many players abuse. In addition, her 1,1,1_4 mixups is taken away, as flash will interrupt after the 2nd attack. Shutting these types of things down is going to make life much easier for you and will throw your opponent off their game once they realize they can't use something that they used to abuse.
Now, when it comes to annoying pokes, flash can interrupt a lot of jab strings and such as we all know. When I use flash, I tend to throw it out when I know I'm safe framewise (or at an advantage) and the range is up close. But another thing that is insanely frustrating for opponents is when their poke strings become flash baits ON HIT. Lars' db21 on hit? Flash will interrupt a number of his options. Same with some generic 1,2 punches. Just keep this in the back of your mind and experiment in casuals to find more.
I also highly advise you to play with 4~1. This has been a lifesaver vs nina players that abuse df1,2 to no end. I found that the regular flash seemed to be just a hair too short when it comes to range, but 4~1 will give you the outcome you want. Flash is probably the most hard to understand move that I've come across, but also one of the most usefull. USE IT, make the opponent fear it, and they'll rethink their aggressive pitbull attitude towards yoshi.
This, in my opinion, is yoshis strongest point. To be honest it disheartening when I see Yoshi players end combos with B! b2,1 because you're KILLING yoshis options for followups for a few minor points of damage. The main benefit of yoshis oki is that you're able to somewhat train the opponent to react to how you want them to, and yield better followups for an extended combo or massive damage. To do this, I end my first post B! combo with uf1+2~ d. This attack hits grounded and techrollers will whiff vs backrollers), but you need to observe how the opponent reacts. If you notice the opponent is tech happy, you can go for the 'Tenshi Special' using bb+1+2, which will connect vs techs for massive damage.
If the opponent refuses to tech and stay grounded post B!, you may opt to dash, d+1+2, 3+4. Players who refuse to tech will sometimes tech after this attack out of panic, thus yielding a free sword sweep for an additional combo. In the event they lay still again, I may opt for uf+3+4, d+1+2. The stomp will connect, however the flea will whiff... but you will be in BT flea position which has a massive unblockable hitbox. If BT flea isn't your thing, you may opt for a simple u+1 for some extra damage. In the event they backroll, a well timed CD1 will relaunch for more damage.
With all these opportunities, why would you even want to b21? Well, I can think of one instance where b2,1 is a viable option with additional oki potential: that instance is where you can end combos with b2,1 ~ kin f+2. I personally rarely end with kin f+2 after b21, I prefer to go for KIN or DGF mixups. If you've found your opponent is backroll happy, give this a shot:
u+3+4, f+1+2 B! b+2, 1~ kin 2, 1+2. This catches backrollers and take a huge chunk of life and leaves the opponent in B! state close enough for even more oki using the same b2,1 tactics. Even if the opponent lays still after b2,1~ kin 2, 1+2 will hit grounded for a chunk of health.
In the event the opponent is techroll happy after this particular combo, I opt to enter DGF after the Kin transition. Now, you're at risk to be floated by a quick ws4 if the opponent techs to a crouch and ws4's, but, if they do not, you have options: Think they'll jab? DGF4 for a CH and another combo. Think they'll stand still? DGF1 for unblockable damage and some frames to play with, or DGF 3+4 for a grab. If they stay grounded, DGF1_3 will make them reconsider their actions.
I figure it would be helpfull to list some combos that I use as staple, so maybe you can benefit (keep oki options in mind for the majority of these):
uf3, b111, b11, f+1+2 B! -- This is great because it realigns and will connect on opponents that are launched BT.
uf3, b11, b111(1)1, f+1+2 B! -- good if you're looking for a longer carry, requires good execution or else you'll be left dizzy.
uf3, b11, b11, db1 B! df3,  -- df3 will hit, and you will be charging for the unblockable sword. Release 1 at the appropriate time and you get some bonus damage and some frames to play with, if you catch a CH, opp is left FDHT with potential to float rollers with df1.
df22_u+3+4, f+1+2 B! b2,1 ~ Kin (DGF) -- see oki for an explanation.
Flash, d22~ bb1+4 to catch techrollers
Flash, d+1+2, 3+4 for oki.
Flash, d+1+2 > 1+2, b2,1 ~ kin f+2 vs bigs. Keep this in mind vs bigs, but do note the delay between d+1+2 and 1+2, if you don't delay, b21 will whiff. Great for post B! bigs that refuse to lay down for massive damage.
df2_uf3, uf4 B! f+3+4, df2, b21 -- for damage, no oki followups
df2_uf3, b11, b11, uf4 B! ssr, f+3+4, d22 for longggggg carries
df2, f+3+4, 1, b1, b2,1~ kin f+2. Saves B! and great damage, so you may opt to not use the kin f+2 if you can get a W!, and B! with NSS df3,1.
1+4, f+1+2 B! -- use the same oki options as the other early B! combos listed (df22, u3+4).