IIRC, when the engine is running, engaging the parking brake cuases the vehicle to hold the vehicle via all four disc brakes - as if your foot was on the brake pedal. Engine off however, the parking brake switches to the conventional handbrake used on other BMW's; a rear drum parking brake that operates seperate from the rear brake disc.

I'm not sure if it's the manual or some other document that states this, I remember reading it somewhere.

The switch between the four discs to the rear handbrake is what causes the movement. There is a bit of roll before the handbrake completely catches.