A varistor is a voltage dependent resistor that starts conducting heavily above a certain voltage. It can not, to my knowledge, be used to control the rotation (speed?) of any motor. And absolutely not the rotation of an AC motor.
I was trying to understand how my 115v AC wench (1500lb load) accomplished the reversal. I have since learned that the varistor is shunting a bridge rectifier and that the motor is actually a DC motor!
