Allows you to maneuver either left or right at variable rates, simply by turning the steering wheel one way or the other to varying degrees.
Okay, but really... it increases the amount of wheel steer per degree of steering wheel movement at low speeds (for ease in parking, etc.) and decreases the degree of wheel steer at higher speeds to help avoid over-steer.
We have it in our X5 and it takes some getting used to.