If you've used medicine balls you'll know that they tend to return most of the energy when bounced. Slam balls differ in that they don't bounce. Because the slam ball absorbs all of the energy you put in to it, you have to work much harder than you would with a medicine ball. Great for plyometric work, where you need a ball that can be thrown without bouncing out of the window or hitting the wall. Slam balls are also ideal for partner work, due to the deformable outer shell.