Moving a column of water to create a flow is what a pump actually does. When water passes through a nozzle, pressure results from a restriction on that column of water.  What we really mean when we speak of a pump’s ability to develop a certain pressure at a certain flow is that the pump is capable of moving that quantity of water per minute through a restriction or nozzle which will create the specified pressure.  

Most pressure washer pumps used in the pressure washing industry rely on a system of cylinders and valves in order to develop a flow of water. The activity of motion of a piston or plunger in the cylinder pulls water through the inlet valve and the upstroke discharges the water through the outlet valve. These valves essentially send the water through the pump by preventing backflow.  

The simplest valve design consists of a poppet and a spring which is hydraulically operated by differences in pressure on either side of the valve.  The valve opens when the pressure against the valve starts to exceed the ability of the spring to hold closed the poppet.  The valve is held closed and prevents flow of the water in the wrong direction on the opposite stroke; This is when the water presses the poppet against its seat.

There are three ways the flow of water through the pump can be controlled: By lengthening or shortening the stroke of the piston or plunger, by increasing or decreasing the speed at which the pump operates or by increasing or decreasing the bore of the cylinder.