Nah, it's not a "secret". Indeed in Tekken 5.0 and DR when Yoshi did f+3+4~b, the announcer said "oops!". Probably because the animation was taken from Tekken Tag (animation of tagging while running), and "oops" would mean something like "but you don't have a partner!" or "you can't run away"
At least I understand it this way.
It's no longer the case in Tekken 6.