When I first started in RC, this was pretty confusing, and no manuals seemed to explain it, they just assumed you new what a "BEC" was and how it worked with the receiver. The power and ground pins for the receiver channels are all tied together. This means you can power the receiver by connecting a battery or regulator (aka BEC = Battery Eliminator Circuit) to the power/ground pins on any of the receiver channels. Many receivers have an extra connector on them just for a battery connection, but again, you can plug the battery or regulator into any channel to power up the receiver.

Most ESCs have a regulator or "BEC" built into them. The BEC takes the higher battery voltage and regulates it down to about 5V to power the receiver and servos. So the throttle connector from your ESC to the receiver actually powers the receiver and servos. The servos plug into the other channels on your receiver, and they receive their power through that connection. Since the ESC controls the power to the motor, it can cut off motor power when the battery voltage gets too low, and conserve power for the receiver and servos. This allows you to maintain control of your airplane and "dead-stick" land it if you run the battery down too low.