The SIT system works by inflating tires as the car moves. It only has two components, a chamber inside the tire wall and a managing valve, and it functions on the principle of a peristaltic pump. A short explanation from the manufacturer:

The tube chamber is kept closed at its lowest point by the normal tire deformation caused by the weight of the vehicle.

As the tire turns against the road this closure moves along the tube chamber, forcing more air into the tire with each wheel revolution. Simultaneously, it pulls outside air back into the chamber from the other side.

The managing valve ensures that the pressure is optimal by alternately letting outside air in and circulating air between the tire and the tube chamber. The managing valve can be either electronic or purely mechanical. The tire pressure can be set during regular maintenance or even by the driver.

You can see an animation of the system here and a video of test results here.

The system is currently being previewed at the 2008 SAE World Congress in Detroit, so we’ll probably hear more about it soon enough.

via.