my car jerk even on a flat land, seems to jerk when i open the door, or when i shut the engine off

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.

Its the same as the E53 theres been a few posts on it under the other forum, its common with most autos.

It's common on E53 Manuals!

I've always put it down to the fact that the parking brake is a seperate drum brake on the rear axle. So when you apply it and take your foot off the main brakes (Disks/Rotors/Hydraulic) it needs just a little movement before it grips. My E39 was the same too. But I could be completely wrong here!

You are not wrong, this is why you'll get a slight movement when releasing the main brakes.