It is the same principle of a jet turbine, it can push a lot of air really quickly.
Currently I only have 1 6990, but it is obscenely overclocked (aswell as the CPU and RAM)and still working on getting it faster. Currently I am averaging around 770-800Mh/s but constant spikes well above 1200Mh/s. The computer will start dumping rejects if I do not keep a window open to this room, even though the house is cooled by a commercial 5 ton AC unit.