you have a box inside the tank and a box on the outside of the tank and a u tube joining the two together.If you set it up over a bucket fill both boxes,put some air hose into the u tube then whilst sucking the air out of the tube keep filling the boxes,once all of the air has been removed from the tube and you stop filling the boxes the water will stay in the tube because of the construction of the seperator in the external box and the grooves in the internal box and the tube opening is below both water levels, the height of the water is exactly the same in both boxes,when water then flows through the grooves and into the internal box(as if you've just turned your pump on)the water level rises and this in turn rises the water in the external box causing the flow through the tube.try this out to help show you the syphon.