HOW: Start on the inside of a door frame or wall. With your elbow bent at your side to 90 deg, push outwards into the doorway with the back of your hand. You can use a towel to make it more comfortable. Think of rotating your hand outwards, with your elbow staying still at your side. Slowly ramp up the amount of “pushing” that you do, staying just under the point of pain/discomfort and hold that level of contraction.
FEEL: You should feel the muscles in your shoulder working, especially the back of the shoulder.
COMPENSATION: Do not let your elbow come away from your body. Do not shrug your shoulders and don’t use your neck muscles.