With your knees forming a 90-degree angle, kneel on top of the exercise ball so that your hands are shoulder-width apart on the floor, arms straight. Keep your legs and feet pressed against each other as you pivot to the right, touching hip to the top of the exercise ball. Return to neutral and then pivot to the other side. Continue alternating, increasing the speed once you have mastered the basic movement. Eventually, you should be executing this movement very rapidly.