Hi Charles,
Do a search under "split tuning" and you will find more information than you'll probably want.
They are for split tuning when you have a raise and a lower on the same string and they enable you to tune exactly the note obtained when both changes are activated together.
The secret is to
1.tune the raise as normal with the nylon tuner
2.tune the split note with both changes engaged using the nylon tuner on the lower change
3.tune the lower only using the split tuning screw.
hope this helps 
having posted this and re-reading your question , maybe I misunderstood you . Hopwever I'll let the post stand in case it helps!!
------------------
Michael
Emmons LeGrande II D10
Fender Artist S10
Peavey Session 500
[This message was edited by Michael Dene on 18 September 2005 at 08:19 PM.]
[This message was edited by Michael Dene on 18 September 2005 at 08:22 PM.]
