Perhaps this diagram
will explain it better. The intake grate is above the level of diviider between the tank and hob
section. The overflow to the sump is below the level of bottom of the U-tube, but the divider to that section is above teh bottom of the U-tube. The U-tubes are in a middle section that has a contsant water volume. When water stops flowing into the overflow it also stops flowing into drain area and both sides of the U-tube remain under water.