Battery Eliminator Circuit (BEC). This is a feature of some electronic speed controllers. Used primarily in radio controlled models, the BEC can sense the voltage drop caused when the battery has little charge left in it. It then cuts the power to the 'drive' motor in order to provide the 'steering' servo(s) with enough power to quickly bring the model safely back to the operator. In the case of an aircraft, the power to the propeller would be cut but the operation of the control surfaces would be maintained in order to perform a 'dead-stick' landing. Without this feature, all control would be lost when the battery expired, probably resulting in a destroyed model.