You're probably familiar with the ab rollout and its many variations, and you probably know that the lats are involved in the exercise. When you pull the wheel or barbell back from the rollout position, you're using your lats, triceps, and various shoulder muscles along with your core.
The heavy medicine ball roll uses the same idea, but it's much, much harder. You need a heavy, sand-filled medicine ball – 30 pounds or more. The heavier the ball, the harder it is. Start on your knees, with your hands on the ball and the ball beneath your shoulders. Walk it out as far as you can, then walk it back. That's one rep.