My 2007 E61 has hill start hold. That is to say, that when I put my foot on the foot-brake on a hill and the release it, the car does not roll back, but stays automatically held on the brakes until I accelerate forward, then it releases. It also releases after I 2-3 seconds if I do not accelerate.

My question is, does this mechanism work by electric motors applying the rear brakes on a timer ? Or some other mechanism. For example, does the car have classic discs for braking when driving and brake shoe's for the parking brake - as I know some BMW cars have this ? Was just curious in anticipation of changing my rear discs and pads in 6-8 months time and also I can't find any service bulletin online that shows the mechanisms.