Spray Gun and Wand 101
Water is forced up the high pressure hose, through the spray gun that's activated by a trigger, through the wand, and through the spray nozzle at the tip of the wand.
The best spray guns are equipped with a trigger lock that will allow you to spray continuously without fatiguing your hand. This is a very handy feature and it's one that you'll appreciate as you clean bigger projects. Trust me, your hand can get tired.
I would encourage you to buy a commerical quality spray gun or wand since they last so much longer, perform very well & feel so much better in your hand.
A Quick Review of How Pressure Washers Work