HOW: Lay on your stomach on a bench or table with an arm hanging down at your side. Keep your elbow straight and use your shoulder blade muscles to bring your arm up and straight out from your shoulder with your thumb facing up. Once your arm is at shoulder height, return to the starting position and repeat for the prescribed amount of reps.
FEEL: You should feel your shoulder muscles working.
COMPENSATION: Keep your upper trap relaxed, only use your shoulder blade muscles.