Better still! Stop and do this on yourself - squeeze your shoulder gently forward, or even just hold your shoulder still, as you externally rotate.
NEXT, as you do external rotation, really focus on scapular retraction during the movement.
Feel the massive difference? Much smoother and the range is much greater.
Hence, the main focus of the drill should be on timing Scapular Retraction with the movement above. Stop the External Rotation at the point when Scapular Retraction can’t go any further. That’s what today’s video is about!
Or else the exercise is hurting your shoulder.
This client is doing a pretty good job of it (watch the point of the shoulder):