STARTING POSITION:
Select the appropriate resistance in the weight stack. Lie on your back with your feet flat on the floor and facing the pulley. There should be a cable with a handle that is attached to a low pulley. Grab the handle with a prone grip. Your arm should be straight. Contract your pelvic floor and core while keeping your chest up. Retracting your shoulder blades, pull the handle toward the shoulder. Your elbow should be around your shoulder height and the palm should face the floor.
MOVEMENT:
Do an external rotation of the shoulder avoiding any other movement of your body so that your palm is facing the roof. Exhale during the movement. Return to the starting position with a smooth movement, preventing moving weights from touching the rest of the stack. Repeat the movement for the specified amount of repetitions and then repeat with the other arm.
TIPS: The exercise key is quality over quantity. The trick is to perform the exercise using proper form.