From the above-attached System Operation and Component Description document...
Auto Hold
The auto hold feature is activated and deactivated through the APIM (SYNC) message center. For the system to activate, the vehicle must not be moving, the driver safety belt must be buckled and the driver door must be closed.
The Anti-lock Brake System (ABS) module receives the driver safety belt buckle status from the Restraints Control Module (RCM) , driver door status from the Body Control Module (BCM) , while brake system pressure and the wheel speed are received from the ABS sensors. This information allows the ABS module to determine if the vehicle is stopped. Once the previous conditions have been met the auto hold feature can be activated. When the system is activated, the APIM sends an auto hold message to the Gateway Module (GWM) over the High-Speed Controller Area Network 3 (HS-CAN3) and illuminates the auto hold indicator. The GWM relays the message to the ABS module over the Flexible Data Rate Controller Area Network (FD-CAN) .
Once the auto hold feature is activated and the driver presses the brake pedal, the ABS module closes the isolation valves in the Electric Brake Booster (EBB) unit to maintain the current brake system pressure at the wheel ends. The ABS module maintains the pressure until the driver presses the accelerator pedal, shifts the transmission into PARK or after a specific time limit has been reached. The ABS module engages the parking brake after 2-10 minutes, depending on the grade of incline the vehicle is currently stopped on, the steeper the grade, the shorter the time.
Good luck!