Also - if they've visited that forum before - during the next browser session that essentially marks all those posts read up to that point. So when you go into a forum, it's best to read all the unread posts within the forum otherwise the whole forum gets marked as read on your next visit.
(It's forum based and doesn't track the individual posts).
The new post tracking has been reworked in 6.5 and also offers an option to instead start at the top of the thread instead of the first new post for them.
I visit more than a boatload of forums using UBB.Threads - the new post tracking mechinism is generally right on and works better than any other product currently on the market.
