When you do this move, it pulls the shoulder blades in towards your spine. This builds strength in the middle and lower trapezius muscles, serratus anterior and rotator cuff muscles. It's one of the best exercises to train shoulder stability. Notice that we are not using any weight on this to begin with. You might find it challenging enough as it is.

Remember if you are having trouble balancing on the ball, take that element out of the equation and just kneel on the floor.