The Sway Stool supposedly helps improve the posture and circulation of the person sitting on it. The stool's central column (which is located directly under your spine when you're seated), is flexible, solid cork, while the rest is perforated. As a result, the seat flexes and pivots under your weight. Because it moves every single time you do, it helps improve your sense of balance.
The Sway Stool is made of 100% pure cork, which is reclaimed waste material from the bottle-stopper industry. Since it's recycled, you can feel good about buying a green product.