"Screen Resolution" setting - how is it supposed to work? is there any help?

[Chris G]

I'm very confused by the "Screen Resolution" application in
Preferences, when loaded it calls itself "Monitor Resolution Settings".

First question, what does the "Mirror Screens" button do? Turning it
on just makes a blue box appear in the middle of the window with
"Unknown" written on it.  Turning it on also seems to reduce the
choices I'm presented with.

Secondly what does "Detect Displays" do?  

Thirdly where's the Help?  When I click on Help I get nothing.

-- 
Chris Green