Originally Posted by from the BMW Technology Guide:

Servotronic does not regulate the power steering via the engine speed, which is the usual method: it functions instead in conjunction with the speed the car is currently travelling at. The electronic modification of servo pressure depending on speed guarantees impressively high servo power even when travelling slowly; this power is reduced at higher speeds. This means that only minimum effort is needed to turn the wheel at low speed; steering assistance drops continuously as you accelerate, increasing the direct contact with the road surface and enabling you to steer with increased precision. In addition, you benefit from added comfort and ease of movement when parking or steering in confined spaces, for example when travelling through narrow openings and alleys.