I'm not totally sure about the question, but I'll try to answer it as I see it. Once the siphon starts, and water flows down the tube, it will lower the level of the water in the overflow box. Now since the level in the overflow box is lower than that in the tank/prefilter box, it will start a siphon, sucking water from the prefilter box into the overflow box. Also you need to try to get all the air out of the u-tube, as the siphon won't start with it in there.
Never really worked with overflows myself, but I'm working on the conceptuals. HTH