Electromagnetic brake torque control
The device mentioned uses an electromagnet to attract a rotating armature plate inward. Removing the coil power releases the armature to be pulled back by springs. 

Thus, using PWM on the coil will provide a graduated braking torque. 

Points to keep in mind:


Too low a PWM frequency will cause perceptible shudder
IF the PWM frequency is a harmonic of the rotation rate, resonance will occur, with detrimental effects.


PWM-based electromagnetic actuation such as in this case can be done with a spread-spectrum PWM signal, i.e. one where the PWM base frequency varies cycle by cycle, often using a pseudorandom sequence.