How a pressure washer works
The simplest way of explaining it is this. Soap or detergent enters the washer from one hose while cold water flows in from your water source through another hose. A water pump which is powered by the engine draws in the soap and water and mixes them together. Most washers will heat the water and then the pump accelerates the water and squirts it out through a nozzle. All of which give you a high-pressure result.
Most pressure washers will work from a standard hosepipe, but some can take a water supply from almost any clean container. Using rainwater collected in a water butt is a really economical way of getting your cleaning done! The whole point of using a pressure washer is that it saves time so thinking ahead about where you're accessing your water source is a good idea. You ideally want this process to be as convenient and easy as possible