Wayne Dempsey has an illustration of this in his 911 engine book. It removes a arch of material from the cylinders skirt.

The inside of the engine is an air pump, any aid to help the air move around will improve the loss of horsepower. This is in the same catagory as boat-tailing ( angling the face of the case webs) and knife edging the crank.

This was one of the most kept secrets of engine builders. Bruce talks about it and Wayne talks about it. But I have yet to see measurements of how much material is removed. Anybody have a picture...especally a 2 ltr.

thanks
Bob