Downstreaming defined........
A venturi is simply a restriction in the injector that forces the water to speed up when it goes through it. The faster the water goes through, the more detergent/chemical comes out. This is called "venturi vacuum" and is greatest at fastest water flow.
When pressurized water enters the injector inlet, it is constricted toward the injection chamber and changes into a high-velocity jet stream. The increase in velocity through the injection chamber results in a decrease in pressure, thereby enabling an additive material to be drawn through the suction port and entrained into the water stream. As the jet stream is diffused toward the injector outlet, its velocity is reduced and it is reconverted into pressure energy (but at a pressure lower than injector inlet pressure).
When a sufficient pressure difference exists between the inlet and outlet ports of the injector, a vacuum is created inside the injector body, which initiates suction of a liquid through the suction port.
This "effect" can only be achieved when you use a tip that allows the pressure at the outlet to be less than that at the injector inlet. This is why you need to use the larger, less restrictive tips to "draw" soap into the stream.
Everything beyond your downstream injector, becomes a potential for restriction. The proper trigger guns, length of hose and tips all play a part in the outcome of the "draw" rate.
Here is a simple image I did to try and demonstrate.
