Wouldn't the same thing be accomplished by hitting the refresh button? Please accept my apologies for not knowing exactly what this button did, I wasn't even aware of it's existence before I read this.
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.
Perhaps they can compare with the http://www.audioshark.org/activity.php site and figure out why it works there. I briefly compared the HTML source code, but nothing obvious jumps; sounds like it's all in the activity.php page: