In a system that uses inertia sensors, the fuel pump will only operate when the safety circuit containing the inertia sensor is complete. In this circuit there is a fuel cutoff switch which uses a mechanical device to break the circuit if the car is involve in an accident. This inertia sensor consists of a steel ball held in a funnel-shaped container by a magnet.
When the vehicle is subjected to impact shock or if it tilts rapidly, the steel ball breaks free from the magnet's restraining force and rolls up the side of its container, pushing against the circuit-breaking switch, which shuts off the power to the fuel pump – or pumps, if there is more than one. Once activated, the circuit remains in an open position until it is reset.