This lid replaces the standard one on a 19L or 9.5L cornelius keg.
It includes another gas post with a dip tube on it at the end of which is a carbonation stone.
This allows you to connect a gas line and pump CO2 or Nitrogen into the keg and have it bubble up through the liquid from the base of the keg, increasing the speed at which the gas absorbs into the liquid.
Have started to use them down tube too stiff replaced it your silicone beer/gas line much better be another month before i get to start drinking from those kegs with the new lids.