How does a hydrodynamic retarder (Voith / hydraulic transmission retarder) create braking torque on a heavy vehicle driveline?

1. By pressing carbon-ceramic friction pads against the driveshaft
2. By spraying pressurized water against the vehicle tires
3. By creating a magnetic field inside the wheel bearings
4. By pumping oil between a spinning rotor connected to the driveline and a fixed stator housing, converting kinetic energy into fluid shear heat dissipated by the engine cooling system