This one's easy (the answer, not the move). It's all in the technique. I'll break it down for you:
The movement starts from the centre, not the hips as many people think, aim the punch with your body, not your hand. Use your legs to rotate your body around the central line, after you've generated the power with your whole body, throw your shoulder into the direction of the punch aiming to concentrate the power you've generated into the end of your fist. The secret is that you use your body for the power and not your arm, so your body may actually move a great deal while your fist only travels one inch. It takes a lot of practice to time your movements correctly so get training. This is also the same movement as a punch from a longer distance only you need better timing.
"Ahh Shrek! your better warn someone before you crack one off, I had my mouth open and everything"
Sticks n stones'll break my bones, but if I land the first one, you're in trouble!