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 back staying at your side with your palm facing in. Once your arm is at your side, 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.