If you suddenly apply the brakes, the auxiliary brake system will interpret this as an emergency emergency stop and provide enhanced braking to help the driver who cannot firmly hold the brake pedal.
If you hit the brakes hard, more braking force will be applied. During this time, you may hear a sound in the engine compartment and feel the vibration of the brake pedal. This does not indicate a malfunction.
The auxiliary brake system is activated when the vehicle starts to move faster than about 10 km/h. It turns off when the vehicle slows down below about 5 km/h.