California Code, Vehicle Code - VEH § 521

A “retarder” is a device, other than a brake, which, when activated by the driver, applies a retarding force to the wheels of a vehicle without the use of friction.  A retarder may be installed in or on the engine, exhaust system, drive train, or wheels of a motor vehicle, or an axle or wheels of a towed vehicle.  A retarder may operate by altering the valve timing of the engine, by controlling the flow of a circulating fluid, by applying an electromagnetic force, by controlling the release of gases from the exhaust system, or by other means.  A retarder may or may not be capable of stopping the vehicle upon which it is installed.