For some mysterious reason, Echidne has a fantastic tribute to Bruce Lee up on her site, and mentions the famous "One-Inch Punch," which would send people flying backwards with little to no windup. (See the video!) Now I had read elsewhere that even Bruce Lee never harnessed the full power of the One-Inch Punch, and here's a random and obviously reliable website that seems to confirm that:

The One Inch Punch, which was made world famous by Bruce Lee, is in truth an ancient technique in Weng Shun Kuen. Bruce Lee, who was never tutored in this technique, learned it by spying on senior students. Because he never got to learning the second form, "Chum Kiu", that trains the footwork that is needed to perform the One Inch Punch correctly, he never got that part right. In fact, the One Inch Punch can easily be learned, if one understands the principle behind it.

Principle shminciple. How does it work? This part actually seems like a decent explanation:

We all know what it feels like when we have to sneeze uncontrollably. Or when we are startled so bad that our hair in our necks stand on end and you feel this "electrical" tingling in our spine. Now THAT is natural Fa Jing! It has got something to do with that same uncontrollable force that’s unleashed while you’re sneezing. You can’t keep your eyes open, no matter how hard you try. That is how close I can can come to describing the essence of Fa Jing for you.

Ah! Speaking for myself, I never really got beyond yellow belt in my elementary-school karate class, so this is probably out of my league, though I did master a fearsome Tiger Claw. Or maybe it was an Eagle Claw. Nevertheless, fearsome. Also, this, from Echidne's comments, is very funny.