This switch is designed to open the circuit to the fuel pump in the event of a hard impact above certain G levels. It is put in-line with the ground side of the relay that controls power to the fuel pump(s). Modern cars are running higher and high fuel pressures so if there is a breach in the fuel line during an impact, fuel can spray over a very large area very quickly.  When designed G loads are exceeded the switch opens and automatically turns the fuel pump off.

The vehicle must experience a high G impact over a very short period of time, forces way above hitting curbs or bottoming the car, bumping other cars or high braking and lateral forces. We've run these switches in Porsche 962's at rough Sebring and not had a single false-trip. When tripped, they can be easily reset by pressing down on the red button. It's a good extra safety feature to have.