No, you didn't miss the intro -- there was no intro.
I'm glad that you noticed it and like it. Personally, I never gave that feature much thought, but I've come to appreciate this similiar feature on another forum I regularly visit. I have been wanting to get this feature working over here, especially with the activity that takes place in this forum. It's easy to quickly lose track of topics you're interested in and/or have participated in.
This feature was turned on late this afternoon. The feature has always been there, but I wasn't happy with how it worked. So I've been waiting until I could make a couple of changes regarding how the notification process worked.
Originally, with this feature on, while you were "watching a topic" you would be notified every time someone posted to that topic. If you happened to pick a hot topic, left for the day, you could come back to find 15 notification messages for the same topic.
What I did was change it so that it would send out a single notification message (regardless of the number of subsequent replies) until you returned to read that topic. After you re-visit that topic, you'll receive another notification message the first time someone posts a new reply since your last visit.