Sit and reach rotate on a stability ball (Neutral spine stabilization, on stability ball, with upper trunk rotation, arm forward)

Sit on a stability ball with both feet firmly on the floor.
Find your neutral sitting posture and hold this position.
Lift up one arm in front of you.
Reach your arm across to one side and rotate your body whilst you do this.
Hold.
Bring yourself back to the middle, swap arms and repeat to the other side.