Rolling the Ball Palm is an excellent exercise for your torso. It is also the first posture that requires you to change hand positions as you perform the Simple Change.
Stand on the edge of the circle facing in the counterclockwise direction. Assume the Supporting Palm posture then lift the right arm up and over the head so the right palm is facing down, and the fingers are pointing to the center of the circle. The image is that you are holding a large ball into the center of the circle as you walk around it.