root - I'm curious what you change about their hand break. I understand your arm behind the back to show the "feel" of hip / shoulder separation, but curious about your hand break change.
Timing. Simplest and first attempt. I've found with some kids who are early with shoulder rotation that if I can make them break the hands appreciably later it solves the problem. With many, I have them go behind the back and then after they get that down I;ll find they can abandon that technique and still stay late with the shoulders.
I wanted to clarify this a bit. What I mean is that I have them severly delay the handbreak until almost through the stride phase. Barely enough time to get the arm up into the cock phase. Typically, they will work their way back to a more traditional break without losing the feeling of holding the upper body back as long as possible.