"FOLLOW" AND "MUTE" (turn individual email notifications on and off for a discussion topic)
The Follow/Mute button can be found at the top right of the start of every discussion:
By clicking "Follow," users who are not fully subscribed to a group (ie. maybe they are on digest) can choose to get immediate email updates for this specific discussion topic.
In an opposite way, by clicking "Mute this Topic," users who are fully subscribed to a group but want to stop getting emails from a specific (perhaps annoying) thread can choose to mute that topic.
Note that the single button switches back and forth, like the on/off button on an old television. The label on the button shows you what will happen the next time you click it.