This is an optional Traxxas "Long" RPM Sensor, and is intended for use with the Traxxas XO-1 1/7 Scale vehicle. To collect critical RPM data (Revolutions Per Minute), a "trigger magnet" is installed in the spur gear of electric models, or the flywheel of nitro vehicles. To offset the weight of the magnet, additional material is molded or cast into the opposite side of the spur gear or flywheel. To read the magnet, a hall effect sensor is installed in the gear cover (electric models) or installed on a support arm (nitro models). Each time the trigger magnet passes the RPM sensor, a revolution is counted. Revolutions per minute are displayed on the tachometer of the Traxxas Link dashboard. RPM is also the key to determining your model's speed. When you select your model in the Traxxas Link application, the app uses your model's transmission ratio and tire circumference to calculate its speed based on RPM, and the speed is displayed on the Traxxas Link App's dashboard speedometer.