The 99152 RPM Sensor is compatible with the Airtronics 92010 (RX-461) receiver that is used with the Airtronics MT-4 2.4GHz FHSS-4T radio control system. The RPM Sensor uses infrared technology to record data from a rotating part , such as a flywheel or spur gear. One black and one white reflective decal are included that is to be attached to the rotating part so the RPM Sensor can "see" it as it passes in front of the Sensor Pickup. When programmed through the MT-4 Transmitter's Telemetry menu, the RPM Sensor can provide direct RPM readings or Speed Readings in either MPH or KM/H to give you real-time feedback that is displayed on your MT-4 Transmitter's Telemetry Screen. This allows you to monitor and keep track of your model's speed while you're driving.