My understanding is that the flaps ( you will note there are a pair in front of the rear wheels as well ) deflect air around the wheels & minimise build up of air pressure within the wheel arches. All making for a smoother flow of air beneath the car, reducing turbulence & aiding downforce.

All respect to a 'proper' expert who may post a more accurate explanation.

 

Its basically an air dam for the tyre, to help provide a more laminar flow under the car. I would assume that it would reduce the air flowing into the arches also. Then there could be a little less wind noise too, and maybe some cooling effects for brakes but Im unable to find definative evidence about air pressures in wheel arches!

 

The S2000 Type S was the first JDM to have them fitted as standard.

http://www.honda.co.jp/S2000/webcatalog/ty...rake/index.html

Translated from Honda JP:

"In addition, also the strike rake which is equipped on front and back, has contributed to the decrease of aerodynamic drag by the fact that the flow of the air which hits against the tire is controlled."

Chris.