Sit on the floor with your upper back against the edge of a bench and a barbell rolled over your hips, padded so it doesn't dig in. Plant your feet flat, roughly shin-height back from your knees, and drive through your heels to lift your hips until your body forms a straight line from shoulders to knees. Squeeze your glutes hard at the top rather than arching your lower back to get there; the last bit of height should come from your hips closing, not your spine bending. Keep your chin tucked slightly so your neck stays neutral instead of craning back. Lower under control until your hips are just short of the floor, then drive back up. Your shoulder blades stay anchored on the bench throughout; only your hips travel.