Each roller gives the balance board his unique character. Vew-Do calls them also rocks. The rocks with a smaller diameter are easier. By increasing the diameter the level of difficulty will get harder. Boards on a simple cylinder will only work in two directions. Rollers with a degree tapered or a radial tapered roller will work also from heel to toes. The training board will not go only back and forth. No, the board will also react like a snowboard, skateboard or surfboard and will move from heel to toes.