I was going to write about this, but aejones says it better.

Since we're already quoting 2p2 posts, I like this quote from that thread:

Originally Posted by KRANTZ

when there's a nuts or air situation, and you can bet something like the nuts that's only slightly better than their best bluff catcher, that just makes you a whole new level of dangerous.

In general the "value bluff" concept is those situations where you don't know if you want him to call or if you want him to fold.