1st variation Lie on the ground on your back and bend your knees bringing your heels almost to where you can touch your hands. Pushing off your heels, bring butt up towards the sky, and leaving only your shoulders and heels in contact on the floor. There should be a straight diagonal line from your shoulder to your knees.
2nd variation Push up with only one heel, keeping the other leg off the ground and pointed straight up, or in line with the other leg. Try to keep the core stable and hips straight.
3rd variation Do the first variation, but as your butt pushes up, twist your weight over to one side alternating reaching over your shoulders.