Hip snap is not unique to Taekwondo or karate; boxers have used it for over 70 years. Sam "The Tar Baby" Langsford told his young boxers to "get them hips into your punches" and Jack "The Manassas Mauler" Dempsey often referred to the importance of the waist twist when teaching his "shoulder whirl" technique.
Hip rotation is the driving force behind hand and leg techniques, but it is especially true for kicks. Many kicks, such as the crescent kick, are useless if the hips do not drive them. Always concentrate on hip movement during kicks and strikes.
In the Theory of Power section of his manual, Encyclopedia of Taekwon-Do, General Choi Hong states that maximum kinetic energy of force is obtained from maximum mass and speed and it is all important that the body mass be increased during the execution of a blow. He thinks that this may be accomplished by combining two methods, hip rotation and the sine wave principle. As will be explained below, hip rotation adds to power, while the sine wave principle is based on false logic and is wasted motion.
How to Perform Hip Snap
When performing hip snap, think of the motion more as a delayed hip movement than as a rotation around the vertical axis of the body. For example, when stepping forward in a front stance with a straight punch, instead of thinking about hip snap as a rotation of the hip during the punch, think about the hip lagging behind the leg during the step and then snapping back into position just as the foot touches the floor.