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.
You probably never thought about it, but most people can't produce as much force using two bilateral limbs to perform an exercise as they can if they perform the exercise with each limb individually and then add together the force of each side. It's called the bilateral deficit and you should take advantage of it to pack on some muscle.