What is the working principle of a Hydrodynamic Retarder (e.g. Voith or ZF Retarder)?

1. It rubs two large wooden brake shoes against the drive shaft
2. It sprays pressurized water onto the exhaust pipe
3. It converts kinetic energy into heat by pumping oil between a spinning rotor (connected to the driveshaft) and a stationary stator, transferring the dissipated thermal energy to the vehicle cooling system
4. It reverses the electric alternator current to spin the engine backwards