How does an electromagnetic eddy-current retarder (Wirbelstrombremse / Telma) generate deceleration torque?

1. By forcing friction shoes against the transmission output flange
2. By energizing electromagnetic stator coils to induce opposing eddy currents in rotating steel rotors on the driveshaft
3. By redirecting hot exhaust gases back into the cylinder combustion chambers
4. By pumping hydraulic oil through a variable constriction orifice