Understanding Forbidden zones in Smith chart

Hi,
I am trying to understand the concept of Forbidden region in Smith chart (Ludwig/Bogdanov, 2nd ed, pn-431). After playing around with ZY-smith chart, I could verify the points they made. But, Still I don't feel confidence. Particularly, given a source impedance (and Z0) how could I quickly draw forbidden zone for a particular matching network (say any of L-match)?
Detailed explanation or any link to 'nice' tutorials will be greatly appriciated.
Thanks.

http://www.qsl.net/va3iul/Files/Impe...g_Networks.pdf

Thanks RiffMaster, but that couldn't help me much.

I saw one book on impedance matching in ARRL library
Why don't you check it if have access. It will have explanation.

Also The link given above is good. just follow it based on your application & need.
ALso another link
http://www.rfcafe.com/references/electrical/smith.htm