Almost all electronic speed controllers (ESCs) under 60 amps or so have a built-in battery elimination circuit (BEC). A battery elimination circuit cuts power to the engine while continuing to send power to the receiver and servos in order to allow the pilot to land safely once the flight battery gets low. On electric power setups of significant weight (5lbs. or more) or complexity (5 or more servos), a separate receiver battery is used so adequate current can feed the receiver and servos without overloading the built-in ESC on the speed controller. External BECs can be used for greater current delivery, but a separate receiver battery is considered for safety so control can be maintained in case of ESC failure during flight.

That's more than you probably needed to know, but talking about BECs may clarify a few things for you.