ADB-X: Automatic differential brake on all-wheel drive vehicles
WARNING: This page does not describe the selected car, but rather 45 other vehicles, including the 2016 BMW M5, 2016 BMW 550i xDrive, 2016 BMW 550i, 2016 BMW 535i xDrive, and 2016 BMW 535i. However, it is still accessible from the selected car via links, so may be relevant.
ADB-X offers the functionality of a differential lock. When a wheel displayed signs of incipient traction loss it is braked automatically, allowing the remaining wheel on the axle to continue delivering drive torque.