Cold-aisle containment stops the cold air within the data centre mixing with the hot exhaust air from any equipment. It forces the cold air through the equipment, where it is heated and then exits into a hot zone. The air conditioning units then take air from the hot zone, cool it and feed it back into the contained aisle (cold zone). This simple technique can reduce cooling bills by over 20%.
The picture below shows the contained aisle at the USDC. The roof panels can be clearly seen, and at far end of the aisle, the containment doors have been shut. The three vertically seperated doors in the image are secure thirds of a rack for the co-location service and the secure half racks can be seen on the opposite side.