I'm on a public computer right now so not much I can do right now but the image is just a link from here
http://image.circletrack.com/f/techa...4_aero_5_z.jpg
The basic idea is that the air coming off the front of the car (to the sides) has some lateral momentum and may, in some designs, largely skip over the wheel well openings. This came up in discussion about the diverters I put on my car a while back (you can see pics in my album).
I seem to remember MetroMPG testing front wheel skirts for his metro and not seeing any improvement IIRC.