Subscribing is different than just the "MY FORUMS"
When you subscribe to a thread you will be automatically notified via e-mail each time a new post is put up in that particulat thread. "I think"
I clicked on the "subscribe to all threads I post in" once. All I have to say that is don't click on this one unless you like getting LOTS of e-mails.
It didn't take long to uncheck that option.