BMW's definition of Servotronic

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.

M Servotronic.

M Servotronic is a special form of power steering geared not to engine speed, like conventional power steering systems, but rather to the car's actual road speed. In practice this means ample power assistance when parking and manoeuvring at low speeds, followed by gradual reduction of power assistance as road speed increases to ensure greater direct contact with the road at high speeds and, as a result, an even higher standard of steering precision. Using the sports switch, the driver can also choose a second steering control curve for even sportier feedback from the steering.