I believe it limits the range of motion on the accelerator pump thereby reducing the duration of the "squirt." It is intended to reduce the off-idle hesitation common on FCR carbs. There is also an inexpensive mod that produces the same results - http://4strokes.com/tech/ktm/crockbkm.asp.
I e-mailed them last week to double check if it would work on the YFZ before I ordered one but haven't gotten a reply. From what i've read the pump squirts a stream of fuel stock and the R&D plate atomizes(sp?) the fuel so its more of a mist than a stream.