What is an auxiliary retarder (ralentisseur — e.g. Telma electromagnetic retarder or Voith hydraulic retarder) and what is its primary benefit?

1. A driveline braking device that slows the vehicle without using wheel friction brakes, preventing brake overheating on long descents
2. An electric motor that accelerates the vehicle up steep inclines
3. A device that increases diesel fuel injection pressure during overtaking
4. A steering damper that prevents tyre puncture deflections