A great checkpoint for the movement of the trail arm during the backswing is looking at it from the down the line camera. Good shoulder movement will keep the trail elbow close to in front of the trail pec. For a right handed golfer, this means your right elbow will be just on the edge of your right pec. If you can see a lot of space between the right arm and the pec, then your shoulder is going to have a hard time externally rotating during the downswing.