bird - 6:58 pm on Jan 9, 2002 (gmt 0)
RewriteRule !^http://[^/.]\.your-site.com.* - [F]
This means that the rule would theoretically be applied to all requests that ask your server for a page from from a different domain than "your-site.com", given that they show the www.iaea.org referrer. In other words, the pattern probably doesn't do what its author had in mind.
Reality, however, is slightly different. ;) The string passed to the RewriteRule only contains the path component of the URL without the hostname. This is the reason why the technically pointless pattern still gives the desired result and simply denies any request where the RewriteCond matches. The rule will by definition never see a string that starts with "http://", but only strings that start with a "/".
If in doubt, I'd simply lump the RewriteCond for iaea together with the others in the upper list and get rid of the second RewriteRule. The "^.*" of the first RewriteRule acheives the same result in a much simpler was, by saying "apply this rule to URLs that contain any sequence of characters, or none".