Can someone rephrase ! a commonly used sensor is the electromechanical "gas dampened ball and tube" design. the sensor is nothing more than a small tube with a switch at one end and a gold plated steel ball at the other, held in place by a small magnet. when the sensor receives a hard enough jolt to knock the ball loose from the magnet, the ball rolls down the tube, hits the switch and closes the circuit. the tube is slanted upward so the ball should return to its original position after an impact.