Varistors change their resistance loads depending on the voltage coming into it. the hot wire is connected to ground, and when in normal voltages, they’re very high resistance. electricty follows the path of least resistance- similar to water flowing down hill.

so, when the voltage is normal, the power goes into the plugs does it’s thing. when a surge hits, the MOV’s loose resistance with the increased voltage, letting it flow into the ground wire instead. This brings the voltage going down the hot wire to normal, until the MOV’s go back to being highly resistive, and normal power.

basically, think of it as being a water wheel, when the flow of water is ‘normal’, the sluice gates are open and the water goes down the sluice driving the wheel. when there’s a lot of water coing in, though it spills out and doesn’t make the wheel spin too fast.