Hi,
Basically the problem is getting the current into your receiver without causing the receiver bus voltage to drop excessively. Standard R/C connectors are good for about 2.5A each and the battery connectors carry the sum of the current of all the servos. When you do maneuvers that call for a lot of servo action your receiver bus voltage drops considerably causing your servos to slow and produce less torque. If the voltage goes low enough it can cause your receiver to brown out and you to loose your connection to your plane. The PowerExpanders/Systems avoid this by providing a high current path from the batteries to the servos through high current Deans Ultra plugs. With the right wiring between the batteries and the PowerExpander/System you will loose basically no voltage to the servo outputs. Also the receive has its own current path and regulator on the PowerExpanders/Systems so the receiver is isolated from all the servo noise and current draw.