The middle wire (red) is the power wire in the servo/receiver/ESC system. The yellow, white, or orange wire carries the control signals. The black (sometimes brown) wire is common.

When an ESC with BEC is connected to a receiver, power for the receiver is delivered through the red wire. Then the receiver further distributes power to the servos. When you disconnect the red wire, control signals still pass from the receiver to the ESC through the yellow, white, or orange wire. But then you must use a separate battery to provide power for the receiver and servos.

FYI, by using the middle wire as the power wire, no harm is done to any of the components if you mistakenly connect a servo plug backwards. The servo won't operate of course, but at least it won't be damaged. Earlier systems which had the power wire in one of the outboard positions could result in damaged components if you plugged in a servo backwards.

- Jeff