acroporidae,
Water in the main tank over flow the partition and flow down the durso to the sump and water from the sump pushes up into the area underneath the sandbed (a 2.5 - 3 inch space is created for this ) The return water pipe goes underneath the sandbed - so it will push (flow upwards) all the detritus from bottom to the top of the overflow - at the same time the heavier detritus will flow to the sump via a underflow created. The entire system is berlin style - as I am firm believer.
And the system is created such that when you need to change water you just have to flush the water from the bottom and the water will reverse (flow downward) its direction which dislogde any detritus stuck in the sand bed.
Hope it is clear.
Thanks Jim & Lester for the kind words