External Rotation - Machine

Set the cable with one handle at elbow height. Grasp handle and pull your elbow next to your waist. Pull your shoulder down and set your shoulder blade back slightly. Your palm should start facing your stomach.

Keep your upper arm and shoulder still as you rotate the upper arm within the shoulder socket. Release the weight slowly and repeat.

If your cable machine's weight stack does not go light enough for you, put the pin on top of the top plate and use no resistance. Or, you can do this exercise with a dumbbell or tube.

Shoulders 

Infraspinatus, Teres minor

TIP: Keep your upper arm and elbow in place, as if your elbow is the point on a compass-your lower arm is the only part that should move.