The knot used is a tautline hitch shown here:

The stuff sack is in the position of the tent peg in the how too and the load/tension to cause the knot to grab comes from the contents pressing out. Hold the knot and slide it along the standing section to tighten and loosen.

The sack is an Outdoor Research for a Thermarest with one of the recent USMC poncho liners with zipper with a quart Nalgene for scale.

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