These check valves are a really great idea, especially if you are wanting to lay our mini kegs on their side in the fridge for force carbonating/dispensing.
They work as a normal gas disconnect except that unlike others they have a liquid no return valve built into them.
This stops any liquid running back up the gas post and into the regulator which will ruin the internal mechanisms. We have noticed this being a potential issue especially when force carbonating kegs that are lying on their side as if the gas in your cartridge runs out there is nothing at all to stop the liquid entering the regulator.
We definitely recommend getting these if you intend on lying the keg on it's side while connected to gas. Liquid inside the regulator is not covered under their warranty under any circumstance) but they are also an excellent safeguard no matter how you plan on using your keg.
The valve does make a noise each time gas enters with this type of connection, this is a sign that it is working correctly.
This item is exactly what Dave n Jo say it is! Simply attach between your keg and reg and there is no chance of any beer running back to the reg causing it to be permantently damaged. A must have for any brewery!!