Every client that walks through our doors here at Cressey Sports Performance needs to own their ability to externally rotate their shoulder joint. Whether it's a pitcher that throws 95-miles per hour, the powerlifters that bench and squat a ton of weight, or the general population client that just wants to alleviate their pain, shoulder external rotation (ER) is one of our top priorities.
One of my favorite ways to teach good ER is through prone shoulder external rotation end-range lift-offs... try saying that three times fast.
I love this drill because the floor provides the feedback necessary to understand how to truly rotate the shoulder. Because the elbow is making contact with the floor as well, it'll be much easier to feel any movement that isn't coming from the shoulder itself.