Marking a thread as "read"

[Brandon Martus]

and here's some insight as to how and when the icons all change, etc

...from the vBulletin support forums
Whenever you click on any part of the forum, vB starts a 'timer'. If the next click occurs before the timeout setting, then a new timer is started and the new message icons stay 'on'. If however there's no new click until after the timeout setting, then the prvious messages will be considered read and the 'on' icons will only appear for messages posted since the last timer was set.