Can you explain the Ras recruitment system?

[janapix]

Studying yeast two hydrid systems and came across this. I'm having trouble understanding how RSS works. If you could explain or direct me to a good research/review paper, I'd be very grateful.

[janapix]

To clarify my problem, here's a diagram from Current Biology 1998, 8:1121–1124
http://biomednet.com/elecref/0960982200801121


image link: http://i317.photobucket.com/albums/mm39 ... ff/ras.jpg

I understand that mRAS complements mutated yRAS, and that mRAS is fused to the bait, which interacts with a certain switch which is already fused to the plasma membrane. My questions are:
1) is the cdc25-2 yRAS mutant already attached to the plasma membrane, even in the inactive GDP-associated form?
2) How can we tell what the switch is? Can't the bait interact with unindented proteins on the yeast cell membrane? Isn't this LIKELY as well, given that a third of all eukaryotic proteins are membrane-associated?
3) If so, how are false-positives eliminated from this experiment?