When you turn on the fast idle, a solenoid (Skinner) valve applies air pressure to a cylinder that pushes on the fuel lever on the governor. It also applies pressure on an other cylinder that limits the amount the fuel lever can move. That is where you can adjust the desired rpm of the fast idle, usually around 1000 to 1200. The fast idle is interconnected with the parking brake, so it only works when the parking brake is applied.
