A gas check valve is a one way device allowing gas to flow only in one direction. When the gas is not flowing the check valve seals, preventing any back flow.
Gas regulators designed for use in the draft beer industry include standard shutoff valves which do not have a check valve. This is because shutoffs with check valves are not required for the majority of draft beer dispensing applications as all standard keg couplers have a built in gas check valve. This prevents the back flow of beer from the keg into the regulator.
However, homebrew couplers do not have a built in gas check valve. Therefore when using homebrew couplers, it is important to use regulator shutoff(s) with a check valve. This is important because without a check valve, any loss of gas pressure will result in the beer backing up into the gas line and into the regulator.
If purchasing a gas regulator for use with homebrew couplers, be sure to also purchase a shutoff with a check valve so the standard shutoff on the regulator can be replaced.