Pop up making background flash INACTIVE

I have NO idea what the terminology would be for this so I was unable to find anything searching because I don’t know what to search for exactly!

Anyhoo, I’ve this on several sites. You’ve got your main flash working as a main menu on your active browser window. Then you click a link and you get a pop-up. Easy enough. But there’s some code or something that can make your main menu and background totally inactive while you browse within the pop-up. When you’re done with the pop-up, you close it and then click ANYWHERE on the inactivated “main menu” flash to reactivate it. Follow me?

The first example I could think of is http://www.dunun.com.

Load it up, go inside and click the first orange linked text you see. It will bring up and pop-up and “grey-out” the main page.

Anyone know what this is called and even better… how to do it?

Thanks tons…