Cable External Rotation

Trains the Shoulders, Infraspinatus and Teres Minor.

How to do it

Set a cable pulley to roughly elbow height and stand side-on to it.
Take the handle in the hand furthest from the machine and tuck that elbow against your ribs, bent to ninety degrees.
Step away until there is tension with your forearm across your stomach.
Keeping the elbow at your side, rotate your forearm outward and away from your body.
Pause briefly at the end range, then let the cable draw your forearm back across your stomach under control.

Letting the elbow travel away from the ribs, which hands the movement to the rear deltoid.
Setting the pulley too high or too low. At elbow height the resistance stays horizontal through the whole arc, which is what the cable version offers over a dumbbell.
Loading it like a normal cable exercise. The infraspinatus and teres minor are small muscles with short leverage; the lightest plate on the stack is often correct.
Assuming pressing covers it. Nothing in a typical program rotates the upper arm through a range, and external rotation strength lags further behind the more someone presses.