I'm new at the forum and I'm awating my new X5 in June. So I have a lot of time to read the manual which I found in this forum. On Page 57 there is the following information

'With the engine stopped, the parking brake acts on the rear wheels via an electromechanical system. When the engine is running, the parking brake acts on the disc brakes of the front and rear wheels via the brake hydraulics'

If this is correct, there must be a reaction of the car, when having the parking brake set and switching the engine on or off.