I'm not a pro at this but this is how I understand it all to work. You connect a CO2 source to the IN port at around 5 psi and then a gas line from the OUT port to your cask/fermenter. The breather keeps the pressure between the two at atmospheric pressure. Original designed for cask ales which are not carbonated. Normally as ale is drained from the cask, it allowed air in, which is bad for the shelf-life of the ale. The breather allowed CO2 into the cask instead at even pressure. This kept it fresher longer without carbonating it. Using a similar process, some brewers connect the OUT to the fermenter so that when they cold crash and there is negative pressure in the fermenter, the breather equalizes that pressure with CO2 instead of the fermenter sucking up blow-off solution or oxygen. This is what I bought them for (bought 5 to get a good deal and this is the last one I am selling). Note - I have not yet done this but have seen other posts on this forum who say they have and it works as described. In addition to cold crashing I also plan to use mine to equalize the pressure when closed transferring from fermenter to keg. ie - co2 at 5 psi connected to IN port. Out port connected to blow-off tube at top of fermenter. connect drain plug from bottom of fermenter to liquid out port on keg. as fermenter drains, it would normally pull in O2. Instead this will pull in CO2 at equal pressure. Hope that helps.