This is a governor switch in parallel with a resistor. The coils for the motor are in series with that part of the circuit. Based on this video I found, it looks like the switch opens and closes at a certain frequency which changes based on how close the contact points get. Is that correct? Does the use of AC current make that work somehow? I'm trying to figure out the stress on the parallel resistor at lower speeds.

Sorry if this is in the wrong thread, I couldn't figure out where it should go.

 

We have that model, there is a knob on the end of the unit that when turned advances or retracts the contact in order to act as a RPM governor.

 

there is a knob on the end of the unit that when turned advances or retracts the contact in order to act as a RPM governor.

Kind of an analog PWM. With the resistor acting like a snubber for the contacts.