Hip Snap versus Sine Wave
Most martial arts, including most Korean martial arts, including most versions of Taekwondo, use hip snap. However, International Taekwondo Federation (ITF) based practitioners also use what they call sine wave. In the sine wave, the body rises as a technique is in transition and drops at impact of the technique in an action called "knee spring." Though differing in motion, the two methods have the same goal, to add power to the technique. When performing fore fist punches from a sitting stance, hip snappers use quick hip twists to add power by snapping body mass into the punch. The rotation is around the center of mass but the center of mass does not move. Sine wavers raise the center of mass and add power by dropping the body mass into the punch. Snappers vibrate, wavers bob up and down. Some Taekwondo practitioners combine the two motions. They raise and cock the hip and then drop and snap it.
General Choi theorized that the sine wave movement was an effective technique. He taught his instructors that it was an effective technique and they in turn taught their students that is was an effective technique. Over the years, all these people, in an attempt to justify the use of the technique, have rationalized its use in Taekwondo in spite of their being the only martial arts stylists to use it.
In his manual, General Choi, describes three types of waves: sine wave, horizontal wave, and saw tooth. The sine wave is a slight up and down motion on a curve that flows smoothly from one stance to the next, the horizontal wave is a straight line movement, and the saw tooth wave is a straight up and down movement.