Heels on the ball, hips up, fight the wobble.
Press steadily through your heels to keep the ball still and your hips level.
With your heels up on a ball instead of the floor, the glutes have to lift your hips and keep an unstable platform from squirting away, which lights up the whole back of the hip from the first rep. The hamstrings assist heavily, both extending the hip and anchoring the ball through your heels. It is the bridge variation to reach for when floor glute bridges have become too easy but you are not ready or equipped to load a barbell version.
You are driving through your heels unevenly or rushing the lift. Press both heels down with equal, steady pressure and raise the hips slowly, the ball settles when your force does.
Heels on top with a slight knee bend is the standard. Straighter legs shift more work to the hamstrings, more bend biases the glutes, so adjust based on where you want to feel it.
Slow the tempo, add a pause at the top, or progress to one heel on the ball with the other leg held in the air. The single-leg version roughly doubles the demand on the working side.
