The "B" screw changes the distance of the upper pulley from the cassette. When the screw is improperly adjusted, the chain can bounce up against the cassette teeth. Having a properly adjusted "B" screw will make for better shifting. I can only describe the "B" screw as being the one that kind of sticks up in the back of the derailleur. I don't know what you're talking about in terms of downhillers playing with this screw. You sure they don't just play with the cable tension?

-Moab