well as Chinese martial arts. For example, Choi observed that the kinetic energy of a strike increases quadratically with the speed of the strike, but increases only linearly with the mass of the striking object. In other words, speed is more important than size in terms of generating power. This principle was incorporated into the early design of Taekwondo and is still used. Choi also advocated a relax/strike principle for Taekwondo; in other words, between blocks, kicks, and strikes the practitioner should relax the body, then tense the muscles only while performing the technique. It is believed that the relax/strike principle increases the power of the technique, by conserving the body's energy. He expanded on this principle with his advocacy of the sine wave technique. This involves raising one's centre of gravity between techniques, then lowering it as the technique is performed, producing the up-and-down movement from which the term "sine wave" is derived. The sine wave is generally practiced, however, only in schools that follow ITF-style Taekwondo. Kukkiwon-style Taekwondo, for example, does not employ the sine wave and advocates a more uniform height during movements, drawing power mainly from the rotation of the hip. The components of the Theory of Power include: Reaction Force: the principle that as the striking limb is brought forward, other parts of the body should be brought backwards in order to provide more power to the striking limb. As an example, if the right leg is brought forward in a roundhouse kick, the right arm is brought backwards to provide the reaction force. Concentration: the principle of bringing as many muscles as possible to bear on a strike, concentrating the area of impact into as small an area as possible. Equilibrium: maintaining a correct centre-of-balance throughout a technique. Breath Control: the idea that during a strike one should exhale, with the exhalation concluding at the moment of impact. Mass: the principle of bringing as much of the body to bear on a strike as possible; again using the turning kick as an example, the idea would be to rotate the hip as well as the leg during the kick in order to take advantage of the hip's additional mass in terms of providing power to the kick. Speed: as previously noted, the speed of execution of a technique in Taekwondo is deemed to be even more important than mass in terms of providing power.