RE: Futaba T4EXA
Smaller ESCs are equipped with a BEC device, a Battery Eliminator Circuit. Should be described in the instructions, but in a nutshell, it regulates the voltage down from the main battery to power the receiver and servos.
There's also another TLA (three-letter acronym) related to ESCs, called the LVC. This Low Voltage Cutoff cuts power to the main motor so it doesn't completely discharge the main battery, leaving you no control at all.