I'm not sure about Timbuk2 bags, but usually you form a loop with the excess strap at the buckle. This generally allows you to quickly add or remove slack with one hand. I don't know if the Timbuk2 buckle allows you to do the one-hand thing, but I would think the same principle applies in terms of being able to cinch up your bag as your load changes. Here's a pic showing the loop underneath the left arm of the rider on the right:

http://www.flickr.com/photos/onezilla/537297262/