I looked online earlier and found a forum posting from February that explained this is a feature that premium publishers are allowed to use when their ads aren't being targeted as well as they should be. And Google must approve of its use.

It is not a standard feature. It may be a "premium" feature but I know others have been able to use it *after* being invited by Google to do so or by getting permission to do so from Google.

It would be fun to play around with.

I have posted about it now on http://shiwej.com