Punching versus Bullwhip
When punching, think of the arm as a bullwhip. When snapping a bullwhip, the handle of a whip moves relatively slowly but the tip moves very quickly. If the snapping motion is done properly, the length of the whip stays relaxed until the end of the motion when the handle is jerked backward. At this moment, the tip of the whip will exceed the speed of sound (break the sound barrier) and make a popping noise. A punch should be performed in the same manner. The arm stays loose and relaxed as it moves toward the target and, at the moment the fist contacts the target, the arm is jerked backward, which increases the speed, and thus the force, of the punch.