Think of a titty fuck but instead of a broad's tits, you're using a dude's pecs. Simply put, dude #1 is fucking dude #2's pecs, which dude #2 pushes together so dude #1 can fuck 'em. Dude #2's gotta have a nice set of pecs though, else it'll be the same as titty fucking a chick with a rack of A's.
