Make the thumbs up sigh with your left hand. "Apply" # 1 pressure point of right hand heel (of the right hand) against thumb of left hand (extensor action - stetches left arm making it remain straight.
Now raise the left arm up ONLY by bending the right elbow. Make sure the right wrist does not cock. The left wrist has to cock, and you DO NOT have to cock the right wrist to perform the action (no need you are using your elbow).
I'll suggest you are just in the the habit of cocking your right wrist, instead of using only your right elbow. Your brain is associating moving the left arm with a right wrist cock. It does not have to be that way. UP can be provided only by the right elbow. (Almost certainly, the correct action will shorten your backswing and feel a little strange - it seems the whole "get to parrallel on the backswing myth would almost encouge a cock of the right wrist).