What is the function of a vehicle’s retarder?

To reduce the vehicle speed, without using the vehicle’s braking system, e.g. during a long descent.

Assist the parking brake (hand brake) while parking on a steep descent.

Serve as an alternative to the service brakes (foot brake), in order to stop the vehicle when it is under imminent danger during an emergency.

Balance the engine’s function and regulate the motor rotations,