One of the most important parts of any pressure washer is the unloader or bypass valve as this releases the pressure from the high pressure side of the pressure washer when the gun is closed.  This is done with the water which would be going into the high pressure circuit taken and recirculated
In medium pressure or domestic pressure washers the unloader or bypass valve recirculates the water and on automatic shut down models, shut down after a perion usual about 30 seconds, on higher pressure pressure washers there is often a bypass pipe to give some water body to the recirculation circuit and in very high pressure pressure washer there is a use of a bypass reserve or surge tank especially with use of a chemical system.