It doesn't work quite as well as some forum software in that it doesn't keep track of which individual posts/pages you have read.
On that same topic when I used the new link on the
board index it took me to page 2 as I have not been following that topic. I then from the pages links at the top selected page 4 (i.e. I skipped page 3). I then went back to the board index and clicked new again. If it had been keeping track of the individual posts/pages it should have taken me to the first post on page 3, but it didn't - it took me to the first post on page 5.
This means it keeps a record of the last post you have 'read' (if there are more pages that will be the bottom post on the page even if you haven't scrolled down far enough to read it). When you click new against the topic it will take you to the post following the last 'read' - even if that post was months ago. I suggest that if you think of the button as being 'unread' rather than 'new' it may make more sense of the way it behaves.
Edit:
Also, unlike other forum software, there isn't a button to mark the whole topic as read - just a mark unread button which marks all the posts on the current page as unread.
If you don't want to go through all 20+ pages just click on the last page number in the list of pages to go to the last page and that will mark the whole topic as read.