When you are reading a topic ans someone posts a new reply during that time, a notification appears at the bottom of that thread. When you click the notification, the new posts appear.
More info (Dutch!): http://gathering.tweakers.net/forum/list_messages/1451044
I like the feature very much because it will result in a lot less traffic on the populair threads and it is more user-friendly because it's not necessary to reload the whole page.