Think of it as pressured gas piping. On the power company end, there is a piston that continually moves back and forth in the pipe, forcing air out and then sucking it back in. On the other side, there's a giant balloon. At your outlet, there's a valve. Normally that valve is closed. Since the power company can't pull back and create a vacuum, and because it's far easier for it to just be sucking the air from other places where it is connected to the reservoir, nothing happens while the valve is closed. But when you plug something in, it connects the two pipes together, running through your device. Now all of a sudden, both pipes have air rushing back and forth. But if you were to manually open the power company side, you'd have air rushing out and being sucked back in. On the other hand, if you opened the balloon side, nothing would happen.