i have 2 manually operated ones, well they are levers that you either open or close, or find a sweet spot somewhere in between.
there are 2 of them, and completely eliminate the exhaust, and can be closed of course for stock sound.
here is the link to what mine sounds like
http://video.tinypic.com/player.php?v=4ozu51h
another member on here has the electric ones, hell chime in hopefully. thats probably the vid. that you saw.