Pressure washer pump - What exactly does the Unloader do?

Dump off excess pressure when you are not spraying

It's a relief valve, usually adjustable by changing the compression of a spring. The spring force counteracts the water force to allow the output of the pump to be routed back to the inlet of the pump once a certain water pressure is reached.

Even though it is adjustable it should be left at factory specs. A idled pressure washer reaches this full pressure setting and routs all the pumped water back to the inlet. This process creates a lot of heat at the unloader that will be transferred to the pump. Therefore it is advisable to not run a pressure washer on idle for any longer than a few minutes.