Take a look at the bottom filter plates(white in colour),theres a void, here there is a vacuum created by a right angle piece at the far end settlement chamber which is attached to the long black pipe across the (top) back of the tank.The down pipe(out of the picture) has 3 airlines that run vertically down the pipe in the same way as a normal uplift on a undergravel system which generally only has one airline to run it.Water is forced up the tube by three powerful airlines causing a vacuum under the plated beneath the media,the two drilled pieces of clear perspex are where the heavy solids gather for easy cleaning when syphoning.
Hope this clarifies things,its a little difficult to explain.
Really simple system,you can place heaters in either settlement chamber also which eliminates heater burns.