As Unknowns heart stops beating, a small bomb inside of him goes off, sending splintered bone fragments, guts, pieces of meat and so forth everywhere, ripping everything apart - INCLUDING FAST LEGS!
Hence, Unknowns body parts melts into a red-greenish liquid substans (t-1000-style!), and a minute later, Unknown is back in the game!
"Not fast enough, Fast Legs! But thanks for the compliment anyway!" :p