It’s an accelerometer. Basically you suction it to your windshield, push the “start” button, when it is ready you launch your car and it meaasures your 0-60, 1/8th mile and 1/4 mile times. It is EXTREMELY accurate. These newer models, the SS and the RR have a 3-axis system similar to what airplanes have. I lent mine to a friend of mine and he took it with him to Lancaster. He made 8 passes. Every time except for once, the SS meter was Exactly .03-.04 off,(always higher). The discrepency could be attributed to his rollout, and the one time it was way off, he screwed up the launch, he got on it, and then let off a bit, and got back on it.