This is the sensor for Spektrum's AR7100R receiver that features an integrated RevLimit limiter.

The sensor install on the backplate of the engine, eliminating the hassles of installing magnets in the fan or clutch bell and mounting hall-effect rpm sensors on the engine or frame.

Note: Can also be used with many common governors such as Aerospire MultiGov, Align (with red/black wires switched around), CY ATG, Model Avionics Throttle Jockey, and probably many other governors too.