I'm no expert, but from my life experiences, one hit knock outs are a combination of your ability to properly throw a punch, and you're opponents inability to recieve one. Even a relatively weak puncher can knock out a guy with a relatively good chin given the right circumstances. I'd say the great majority of the one hit KOs are on counterpunches. When your opponent is coming in with his chin up, and you land one clean on his jaw, no amount of training will stop those lights from going out.