It is sort of like a door knob that is placed on the rim of the stering wheel. You grip it with one hand and use it to turn the stering wheel. The knob will face you and is made to rotate so you can get a good grip on the wheel.
Makes it easy to back up the car and drive with one hand while drinking the beer with the other.
I think the government made it illegal in the US because it might hirt you in a crash.