When is a chair not a chair? When it goes against the very purpose of what a chair is intended to offer. When I sit down I want to chill out, kick back, maybe even put my feet up if I can stretch out. But putting your feet up while sitting on the Inactivité would probably see you ending up in hospital -- it's only got two legs.
Rather than making its occupant comfortable, it actively works to put you in a state of mild discomfort. With only two legs, you're required to tense out and flex muscles in your body just in order to keep the thing upright. It's intended to promote good posture and a light workout of sorts whilst stationary, encouraging movement and "bearable discomfort" for the sake of your health. Just make sure you're wearing a crash helmet before you plonk yourself down in it. [Design Milk]