This is an optional Traxxas RPM/Speed Sensor Wire Retainer. This package includes two parts: the wire retainer for Slash, Bandit, Rustler, Stampede 2WD and Ford raptor models, and the wire retainer for the E-Maxx. The wire retainer installs inside of the sensor-ready gear cover (TRA3792) to properly position the RPM sensor and wire (TRA6522).