MS tried to make the DVD and HDD caddies the structure that directs air through the heatsinks, but sometimes a bit more is needed, especially in custom builds. This shroud closes around your fan shroud and forces %100 of your case fan's air flow over the heatsink. I use it for my chip testing/flashing console, which stays nice and cool with the lid somewhere in my closet, a DVD drive that's been passed around the entire workshop and a HDD that sits on top of it with no caddy.
