Servotronic is a speed dependent support regulator. It makes your steering rack been engaged easier, the slower you ride and conversely.
It is directed by an own ECU. If this ECU's got less or no power supply, power steering support will collapse.
You can identify Servotronic by a little block fitted to the governor head of the steering rack. The block is equipped with little wires an a jack. No wires and jack at the rack - no Servotronic.

Beste Grüße
Tim