Upper back on the ball, balance and burn through every rep.
Move slow and steady to keep the ball still. Finish flat with a hard glute squeeze.
Resting your upper back on the ball recreates the elevated hip thrust position, so the glutes drive through a bigger range than a floor bridge, from deep hip flexion up to a full lockout with the torso parallel to the floor. The hamstrings assist from below while your whole backside works to keep the ball from drifting. It is the equipment-light way to get real hip thrust range when there is no bench or barbell setup available.
For unloaded or lighter work, yes, and the wobble adds a stability element the bench never gives you. Once you want to load heavy, the fixed bench becomes the better platform because the ball limits how much weight you can control.
Across the upper back, around the shoulder blades, the same contact point you would use on a bench. If it is under your neck or mid back, the ball will roll as soon as your hips move.
Your feet are probably too far forward or too close. Set them so your shins are vertical at the top, tuck your chin, and finish each rep with a deliberate glute squeeze rather than an arched lower back.
