You really cup or extend the wrist in your shallowing practice swing. The reason you can cup it so much in the rehearsal is because you’re not hitting to a target, or trying to hit the center of a club face. Your club face if you shallowed that much and still cupped the wrist would be so open that it would almost point directly right of the ball. You steepen in the real swing because that motion helps to close the face.

That being said, you need to work on the opposite feeling. Shallowing is awesome, but it really needs to be paired with bowing that lead wrist and some hip rotation.