@Kekkaishi lol, I was trying to find the word but "henshin" was all I can think of. Thanks for the recommendation, I'll look it up!
Btw, I recall the guy in the video that I watch found a mask in a locker room and when he uses that mask he turns into that muscular men.

A long time ago I downloaded a Japanese video in which the actor is skinny person but then henshin into a muscular masked men, he does bottom. Anyone knows which video is it?