Ed is right that ventilated rotors act as compressor impellers, using centrifugal force to draw air throught the internal cooling passages.
Usually the cooling air that has passed throught the brake rotors exhausts through the wheel and to the outside. That is why so many cars have black brake-dust contaminated front wheels.
Theoretically wheels with directional spoke/rotors would draw air from under the car, both cooling he brakes and improving downforce and thus roadholding. But at 60 mph the typical tire is turning less than 1000 rpm, so the fan blades are turning more slowly than a typical electric fan and the greatest advantage is in impressing one's peers with cool-looking equipment.