What is an auxiliary engine compression brake (such as a Jake brake or retarder), and when should it be used?

1. An engine braking system that converts the engine into an air compressor to slow the vehicle on long downhill grades without heating the wheel brakes
2. A device that pumps additional air into the reservoirs during a descent
3. A gearbox setting that increases pulling power on steep climbs
4. A full replacement for the service foot brake in heavy city traffic