Your center of gravity plays an important part in the principle of mobility and stability. Lower your center of gravity and you will achieve stability; raise your center of gravity and you will gain mobility. The hub of your action is at you midsection. When your center of gravity rises, you lesson your stability and increase you mobility and vice versa.
Your mind can also control your center of gravity. Try these exercises for example. Lift your partner up slowly holding him around the waist. If your partner thinks of riding an elevator going upward with high speed, his body will be easier to lift. Lift your partner again. If h concentrates he is riding an elevator goin downward, his body will be more difficult to lift.
For mobility move on the balls of your feet, and when pivoting, your knees should be either above or beyond your toes, and not directly over your heels. For stability, lower your body slightly. Stability is essential in punching and throwing.
Jay, Wally. small-circle jujitsu. United States: Ohara Publication, Inc, 2007.