Here is a description from the manufactuter: "Perfect for those who aren't interested in having to swap their empty Soda Stream cylinder every time. Attaching this adapter to your Co2 tank allows you to fill US/CA Soda Stream Co2 Cylinders. The adapter threads onto a male CGA320 Inlet and provides a Soda Stream Cylinder Tr21-4 connection. The manually controlled bleed valve allows you greater precision when it comes to filling your Soda Stream Co2 Cylinders. For best results you should use a 20LB C02 Tank with a syphon tube installed, or put your CO2 tank upside down."