Finally, the control of when and how things are tracked can be found under preferences - notifications
So for me if I stay in a topic/thread for 4+ minutes it will automatically mark it as Tracked, the same when I post. Normally if you create a topic it sets it to Watching. The main difference is that you will get notifications on any post when you watch a thread as opposed to just being able to see a count of old and new unread replies when Tracking a thread.
Hope all that helps.