I'm guessing your refferring to a turbo charged 5 cylinder. If it's the switch on the rear of the intake manifold, it's the over boost switch. It cuts out the pressure signal to the injector pump when manifold pressure exceeds approx 20psi. If you look, you'll find a small solenoid on the fire wall just behind the pump. The switch activates that solenoid to "dump" the pressure, which in turns reduces the fuel delivered.