For Maddie, the cue is about getting the hands up and back, while keeping a bend in the front elbow, rather than down and back with an arm bar. It's a lesson that hasn't been fully integrated into her swing yet, though. It should be a reaction to the rear elbow pulling rearward (away from the opposite batter's box, not towards the catcher), with a tightening of the scap.
Okay, you said what I was thinking. The whole "rock the U" thing is a result of the scap pinch. I really hope nobody has been teaching to turn the front shoulder down and in actively.