Get onto your hands and knees, wrists under your shoulders and knees under your hips. Brace your core so your lower back stays still through the whole movement — this is glute work, not a back exercise. Keeping one knee bent at roughly a right angle, drive that foot up and back toward the ceiling by squeezing the glute, not by arching your spine. Stop when your thigh is roughly in line with your torso; going higher usually just tips your pelvis and shifts the work into your lower back. Lower with control back to the starting position without letting the knee touch down, and finish the side before switching.