It's because it bounces back at you. If you snap one at it and keep your hand out, all you got to do is hold the knuckle there, and it looks like you hit it again. When you get to punch 3-4 you barely have to move them, as the kinetic energy the bag has attained will do most of the "punching" for you. Just give it an extremely short bop every time it comes back, and boom: you are now Pacquiao.