It used to extend the list for another few pages beyond what it deems new for a user but stopped working a while back. It's a useful feature, often I browse threads and and don't have to the time to finish reading or replying and would like to get back to them later. As it is if the thread hasn't been updated I won't be able to see it under the "what's new" tab. It would be great if clicking on "More Activity" button extended the posts by 24 hrs and at least up to 72hrs clicking more.