In partners, one holds a floor bar against their legs whilst laying on their back in hollow shape, the other tries to pull the bar away and the one holding the bar must try to keep it pulled to their legs (straight arms).
From standing in hollow shape on a soft mat, holding a pole (floor bar is too heavy) against their legs, fall back to mat, holding hollow shape with pole pulled to legs.
Spot the full hip circle, encouraging tight shape, no cast, make sure they are looking at the bar at they go back
After the cast, they are typically afraid to go backwards, so the bend their arms and slide down the bar instead of whipping the shoulders back.
I just stop them from moving as soon as their arms bend and tell them to stop bending their arms.