Doesnt work the way many of us want though. It doesnt leave the checkmark saying you have been in that thread until you go in. Making an actual post gives you the visual clue that you have been in there already so you dont go back into the thread.
If they allowed checkboxes by the actual thread title, and you could pick threads and show subscribed or something that would be cool and a time saver. If on page one there are 9 contests for Ontario only, just click the checkboxes and click subscribed (or the like)