Hill Descent Control (HDC).

Hill Descent Control (HDC) holds the vehicle steadily at a pre-determined speed, helping with the job of brake control so that the driver can concentrate completely on steering. This makes the potentially difficult task of driving down a steep hill or over rough ground easy.

Hill Descent Control (HDC), a driver assistant system in four-wheel drive vehicles, can be activated with the press of a button. When travelling at less than 35 km/h, HDC assists you by reducing you to a constant speed of approx. 7 km/h, or 6.5 km/h when reversing. The brake lights come on automatically to warn any drivers situated behind your BMW. The accelerator and tempomat can be used to vary speed between crawling speed and 25 km/h, but if you want, you can of course move faster or slower: HDC then goes into standby mode. It is deactivated once your speed goes above 60 km/h.
HDC is particularly helpful when driving on changeable, loose or slippery downhill surfaces, such as gravel, snow or grass. Together with ABS, it ensures excellent stability while still preventing the wheels from locking if you apply the brakes on a slope. This prevents the vehicle from skidding and permits you to retain control over the steering.
When HDC deactivates, it goes into fade-out mode, gradually reducing braking force and giving you enough time to again take full control of your speed.