The reason this helps cool your case is because by sealing that off you let the fans do the work of creating a vacuum effect in your case. You can also help the air flow by adding in a slot cooler. You can check our review of one here. Running at faster clock speeds then ever before, the graphics chips available today are putting out all time high temperature. Chances are that you'll need more help cooling your video card of choice then what it came with, which is usually just a heat sink.
Heat sinks on today's cards are kind of defeating the purpose.
While it is true that they help to cool down the chip, they are usually mounted upside down. As true gamers this is unacceptable because it means you can be severely limited in your ability to overclock your graphics card due to heat-related issues. So short of manually adding a cooling fan, you do have a viable alternative. The good ole' slot cooler. Cooling is not an area that you should ignore or overlook.