All these variations involve getting your shoulders stacked over wrists on a foam roller, pressing to push your shoulderblades around your ribs and toward the ground while driving your mid-back toward the ceiling. Check in with your press throughout - that's the pain focus of the exercise. It can help to tuck your chin in order to get more space to flex. The first variation is elevated on a pad, yoga blocks, or a step and is less intense engagement since less of your body weight is on your arms. The second has the foam roller on the floor. The final variation you dig your toes into the ground and hover your knees, which dramatically increases the lower ab activation!