As you drink the beer from the barrel it creates a vacuum which sucks the gas out of the beer. When you're about half way down the barrel the beer comes out flat and it may suck air in through the tap to allow the beer out. At this point the beer starts to go flat.
The cylinder allows you to replace the beer with gas, putting top pressure on the beer so the last pint is as good as the first
You screw it on the barrel and give a quick burst of gas. Then take it off.
You buy a full cylinder and you return it when empty. We'll send the replacement cylinder to you when we receive your empty Bottle.
Obviously if you order a few kits you'll get free shipping and save some money!
The post office don't like gas cylinders, you'll have to return using a carrier.