It allows you to preview a link in a window w/out clicking on the link. For example, I can always keep my main page as the "recent posts" search and then view the full thread w/out ever moving to a different page or clicking the link.
Still playing around with it, but so far it is one of the best add-ons I've ever used. Check it out! I have no connection, just a satisfied user.
EDIT: guess I should note that I use firefox, and it appears this may only be available currently for firefox.