Stretch Reflex. If a muscle is stretched and relaxed, and then immediately contracted and used, it will create more force than if it had just been contracted without being pre-stretched. This takes advantage of the elasticity in the connective tissue, The same idea is being applied in plyometric exercises. As applied to kicks, you should first bend the kicking knee completely (chamber). This pre-stretches the muscles on the front of the thigh (quadriceps) that contract during the kick. Hand techniques work essentially the same way. With the arms in a guarding position, the muscles (anterior deltoid, pectoral muscle, and triceps) are stretched and relaxed, ready to contract and attack. Terminology concerning muscles and joints can be confusing. When the knee joint is bent it is flexed and, the quadriceps muscles are stretched. When the knee joint is almost straightened (never lock out a joint), it is extended and the quadriceps are contracted. Think of the joint, and the muscles that move it, separately.
Sequential Application of Forces. Do not unleash all your power at the beginning of a technique. Muscles must act sequentially to coordinate their power to culminate at the movement of impact. The larger slower muscles must contract first with the smaller, quicker muscles acting near the end of the technique. As an example of the sequential application of forces, I like to use the Curl-up (see the illustration below), a creature created by the graphic artist M. C. Escher. When the Curl-up runs, while in its extended position, no matter how fast its legs move it just cannot generate much speed. However, when it curls up, each set of legs sequentially pushes the Curl-up forward and it may achieve a high speed. When performing a Taekwondo technique, if individual muscles sequentially apply their forces to the technique, much more power may be generated than if they applied their forces in unison.
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