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Primarily, the air pressure at the outlet of the duct has to be lower than the inlet. It's commonly known that wheel wells tend to be high pressure . Also the air at the front of the car is high velocity , but low pressure. If poorly designed enough, the duct could flow in reverse. The challenge comes in converting this high velocity flow to high pressure, which is usually done with a plenum. A diverging duct behind the inlet will slow the air down and increase its pressure. It can sound counter-intuitive, but it's how pretty much all ducting works. If the inlet is the same diameter as the duct and there's no plenum, the excess air just spills around the inlet. Sorry for the lecture, been doing a lot of research on this lately for work.
Spats/Gurney flaps in front of the wheel help lower the pressure in the wheel well, so do the vertical fins extending down in front of the wheel. Creating a plenum behind the IC duct to increase pressure on that end also helps. I read a paper about a team trying to do the same thing (cool brakes and driver) and adding a plenum without changing anything externally they ended up having to constrict the outlet because the driver was getting hosed and they had trouble keeping temperature in the brakes.
-Dan
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