One of the biggest helpers for pressure washing you can use is soap.  With the right applicator it is possible to insert any kind of liquid soap or chemical into your wash stream.

Make sure you have all  the right things in place to protect you, those around you, the environment, the machine and the surfaces you are cleaning.  There are many kinds of cleaning chemicals available.  Unfortunately none of them are safe!  So, it is your responsibility to use them in a safe manner.  Read and follow the manufacturers instructions.  If unsure ask your chemical or pressure washer dealer.  If still unsure, don't use them!

There are two main ways to use soap or chemicals with your pressure washer:

LOW PRESSURE SYSTEM
When using a low pressure system you inject soap into your water stream at the low pressure side or inlet of your pump.  Your supply water hose fits into one side of the injector.  The other side fits into the pump.  Through venturi action the chemical is picked up and mixed with the water before it goes into the pump. This means the water/chemical mixture is sprayed on to your surface to be cleaned at high pressure.  

This can be a very effective way to clean but because the chemical is picked up at full volume and goes through you pump, a lot of chemical can be used in a hurry.  Plus, to turn the soap off you need to go back to your machine and turn it off at the inlet.  This makes rinsing a bit of a pain.

HIGH PRESSURE SYSTEM
By far the most popular way to apply soap is by using a high pressure injector.  If confused by the terms used here the names are derived from where the soap is picked up rather than how it is applied.

The soap on a high pressure injector is picked up after the pump, hence the name high pressure.  This injector works much the same as the low pressure injector, by using the venturi principle.  High pressure injectors have a ball and spring arrangement that allows soap to be picked up only when the the pressure in the hose drops.  The venturi action overrides the spring pressure allow the vacuum created by the water moving past the orifice to suck in soap and mix it with the water.  Note that for this injector to work the water volume needs to be maintained, while the pressure drops.  

There are a few ways to drop the pressure to activate this injector.  This is usually done at the nozzle.  By increasing the size of the nozzle you decrease the the pressure, while maintaining the flow.  

The most common way to reduce pressure is through using quick connectors and swapping out the pressure nozzle (small hole) with a soap nozzle (large hole).  Another method is to use an adjustable nozzle to lower the pressure.  Another way is to use a dual wand, which allows the user to lower the pressure by directing all or part of the flow through a soap nozzle by turning a valve on the wand