The C6 DOES have a cinch feature, but it must be different from the C4 (which I did not own).
To cinch a C6, you pull the seat belt all the way out from the reel. That automatically puts it in Cinch, and it will only get tighter after that.
To get out of Cinch, you unfasten the belt and let it reel back in all the way.
To get a good Cinch on the C6, run your seat rearward most of the way.
Fasten the belt, then pull it out fully to Cinch. Let it reel in until it's normally tight. Then grab the belt and pull it tightly across your chest, feeding the extra back into the reel.
Finally, run your seat forward and up to your normal driving position. This tightens the belt some more.
Done properly, you can get the belt so tight that it's hard to breathe. However, I think this is also hard on the seat adjustor mechanism and may lead to the infamous rocking/sloppy seat position, so use some discretion.
