<a href="http://www.flickr.com/photos/ryancecilsmith/3773894021/" title="IMG_1172 by Big Comic Spirits, on Flickr"><img src="http://farm3.static.flickr.com/2423/3773894021_24be5641d1_b.jpg" width="1024" height="768" alt="IMG_1172" /></a>
This is a tensioner, which adjusts the chain, behind the horizontal dropouts. Is this what the guy means, perhaps?