Not a great design
As dry bag compression sacks go, this is not a great design. It does not have a purge valve or an air-permiable membrain to let the air out. It has a drawcord closure abuout 2/3 of the way up the inside of the bag, and the idea is that you put your items in, close the drawstring, and then compress the bag. Finally, you close the roll-top closure to make the bag waterproof. The internal draw string is not easy to use. It is a very cumbersome method. You can not compress the bag once the roll top is closed. The Sea to Summit bags are a much better design.