There are two (2) types of chemical injectors to get the soap into your pressure washer. Downstream injectors allow you to apply soap, and then rinse, without ever having to switch the machine off. This can be achieved with multiple nozzle options, including roll-over nozzles and other double nozzles. This soap is applied at low pressure.
Upstream injectors apply soap at high pressure, and are not typically the preferred way to go because they require a contractor to go back to the machine to turn it off, or have to turn off the machine themselves, and then unhook the soap injector from the inlet. Corrosives in the soap could also ruin the pump or the coils inside of it.