This is just plain dumb. It's physics, ladies and gentlemen.
When a lifter holds a light dumbbell while standing and does some external rotations with that dumbbell, the intent is usually to warm up the shoulder or rehab/prehab the rotator cuff.
The truth is, doing something like this with a dumbbell makes the movement a prisoner of gravity. The force is travelling down, making the biceps the prime mover in holding the weight up – not the shoulders.
In the world of isolation training, it's imperative that we find a resistance that opposes the direction of contraction of the target muscle. In the case of shoulder rehab, the answer will rarely ever be found in the form of dumbbell work. Instead, it'll be found in the world of cables and bands.
Setting up for a true horizontal pull pattern that services the rotator cuff means positioning a band or cable pulley at torso level and lifting from there.
Doing that will make a world of difference that doesn't just fatigue your upper arms or deltoids first.
It's surprising to me that people don't feel the fatigue in the wrong places when they make this mistake and react accordingly.