What is the "Participated Button" Basically you can pick threads you are interested in following up on and anytime a new post goes in one of them it will show "NEW" by the "Participated".
When logged in, all recent threads you have posted in will appear in the "Participated" list (see the button top right of forum under your login name). When someone else posts in one of these threads, there will be a "new" added to show that there is a new post in one of these threads. Once you click on "Participated" or visit the thread with the new post, the "new" will be removed until there is another new post in one of your threads. If you wish to remove this button, go to your Profile, Edit Profile, Settings, Show Participated, choose yes to show it no to remove, click Save Settings.