The master controller is swung to BRAKE. The main circuit breaker opens the traction circuit and closes the rooftop grid in series with the motors. The diesel backs off to idle.
The wheels keep spinning and now drive the traction motors. Current is reversed, so the motors act as generators and fight the rotation — the brake.
The generated current is forced through the resistor grid. Its grid resistance converts the train's kinetic energy into heat — braking force without touching the wheels.
The roof blower spins and the finned sections dump heat to the air. Below about 3 km/h the grid is cut out automatically so the unit doesn't stall.