This Polaris Pressure Relief Valve is threaded on the outside and has a spring inside with a black piece of plastic that keeps it compressed. The spring is Polaris factory set, when the pressure inside the line exceeds the spring tension, the spring opens and relieves the excess pressure. It can be manually adjusted by screwing it in our out of the wall fitting housing. Screwing it all the way in give the Polaris the maximum amount of pressure and speeds it up, unscrewing it will relieve the pressure and slow the cleaner down.