Do you mean the Reefrack with the spray bar design that fits under the liverock?
If so then they water is taken from the top of the tank and pushed down under the rack and through the spray bar. This will then florce the water up through the liverock and giveit higher circulation. It also prevents any build up of waste materials from collecting under the rocks and keeps it in the water colom for longer which alows it to be skimmed, caught in sponges or eaten by the cleanup crew/fish.
Hope this helps.