Coiling places resistance on your stomach muscles as they twist over the hips facing an angled direction toward the serving court. It is a lot like how a turnicut is twisted or twisting a sponge. The middle of the sponge gets the most tension before it is released. I takes time to get this down. But you should feel something tense a little and release in the stomach muscles. The stronger you get in the stomach, the less you feel but you are still doing it.
When you coil in the stomach area it helps push the front hip out a little (hip stretch) which creates a naturally angled knee bend (this is where people think players are just bending there knees to jump) angling your shoulders upward to the ball as your tossing arm stretches the front side. As you begin to uncoil at the stomach it sort of releases the stored energy upward and PULLS your feet off the ground. Your upper body uncoils and extends which begins to cause a jacknifing effect as your butt sort of pushes out or remain in place. In this case, the feet get pulled off the ground - it is not jumping. Jumping doesnt do anything for a serve. Stored and kintecitc energy moving up the body gives you power.
