Okay, so in RE6, the melee system is a tad different from previous RE games.
In RE4, it was simple, shoot them in the face to do one, or shoot them in the legs to do a different one. It worked nicely.
In RE5, they improved upon that. Shoot them in the face for a melee, I'll use Chris as an example for how it worked. Shoot the in the face for his straight punch, when you get them on their knees, there was an uppercut from front and neck snap move from behind. Then lastly if you shoot them in the arm, there is a curved punch from the front and a sparta kick from behind. Not to mention like one move for bigger enemies, like he had the backhand.
In RE6, it's a little different. Does anybody know how each melee is activated? It seems like certain ones are saved for special enemies, and getting them in these postions isn't quite as easy. But of course the head shot does allow a 1-hit kill melee attack, which is always nice. Chris of course throws the enemy right on their head. Press slam style.