From just looking at it I understand how its different. Ingenious I tell you, box that hangs on the back of the tank is longer... so the u tube is longer. With that it mind, once the flow is slowed down due to a power outage or such, the flow slows down and so does the siphon. That means by the time the overflow has sucked as much as it could it comes to a point where the lowest water level it can go in the prefilter box. from there it stops and when the return pump is kicked back on, it starts to work again. its just simple physics if u ask me. im not sure if u understand it or not. simple way to put it is GRAVITY.