I've been wondering about this a long time but Sheldon Brown didn't write anything about it so I figured this would be the best place to ask.

I can see how having less spokes or bladed spokes reduces drag, but can someone explain to me how a deep rim cross section reduces drag? Or how disc wheels and disc-like wheel covers work?

It seems to me that neither of these things reduce the exposed cross-sectional area of the wheel, so i am assuming it has to do something with turbulence... can someone explain the science behind stuff like this? Thanks.

Also, why do pro racers use a disc wheel on the rear but never on the front?