The one inch punch, made famous by Bruce Lee, is actually an ancient technique called fa-jing. The technique is used in the neija (Internal) styles of kung-fu , such as TaiJi-quan and Weng-shun-kuen. Fa-jing literally means "explosive power". Its power comes an "internal" source, as opposed to muscular strength. The technique uses relaxation and creating a fluid, whip like motion that does not telegraph your intentions. The technique surprises opponents and knocks them off balance. The ultimate test of the effectiveness of the one-inch punch of any other technique is "If it works as well as claimed, then why don't professional fighters use it? They could win more fights and make more money!"
There are many ways to generate power in fa-jing. In Tai-ji-quan, it is generated by shaking the waist violently. In Wudang weng Shun-kuen, it is come from the ground. The key is relaxation.
To perform fa-jing, hold your hand horizontally, palm down, the fingers hanging down. Then make a SUDDEN punching movement. The hand should snap into a fist from the sudden speed. Keep arm and hand relaxed, even when the hand is clenched into a fist. Fist should be rotated slightly upward at moment of impact and the arm should not be fully extended. Extend your arm after contact with the target while snapping your body in the punch. Do not push, punch through the target.
Once you have learned the fa-jing Punch, it is fun to demonstrate it as Bruce Lee did. Have someone hold a thick telephone book against his or her solar plexus. Now apply the fa-jing punch through the phone book.