Here's a tear down on how an airbag sensor works. An airbag sensor is responsible for detecting sudden deceleration in a collision. It sends a signal to the airbag computer which use the vehicle speed, yaw, seat belt and ECU to determine if an airbag should deploy in a crash. There's two types of mechanical airbag sensors, the cam type and ball and tube style. The ball and tube style of airbag sensor consists of a ball, in a tube, held to one side by a magnet. When the sensor experiences shock, the ball is dislodged from the magnet and strikes the terminals of a switch, sending a signal to the central SRS computer. The cam type of airbag sensor, explained in this video, uses the momentum of a pivoting cam on its own axis of rotation to strike a switch terminal completing the circuit. A diagnostic resistor is wired in parallel in all sensors. This is for the SRS computer to perform a self check at startup to ensure all sensors are present and operational. This video explains the differences between sensor types and shows how to access and replace your sensor on a Toyota Camry. Procedure will be very similar for other vehicles, as the sensors are usually bolted to the radiator cradle around the headlight area. It also goes into a tear down of an actual airbag sensor, where you can see the little cam mechanism and switch strike terminals. The latest high tech airbag sensors use Micro-electrical Mechanical systems, or MEMs to sense acceleration. Since they're machined or etched into silicon directly, the sensor footprint can be much smaller, in the range of micro-meters.