Whenever possible, you want to mobilize in a position that looks similar in shape to the movement or position you’re trying to change. For example if you’re trying to improve your overhead position, it makes sense to put your arm overhead and mobilize anything that might be limiting your range. In most cases, what you’ll find is that the underam region, which is where the lat and rotator cuff insert into the armpit, is really stiff. Lay on the ground, put your arm overhead, lie on a ball and move around. Chances are you’ll find something that is stiff and sore.
The exercise requires you to have a hard ball like a lacrosse ball, or any hard rubber ball. It will help with overhead positioning and shoulder pain.