Relieves pressure while filtering from one keg to another plus allowing reading the pressure in the receiving keg.Notes: You need to wind/unwind the end of the valve to adjust the pressure. It sometimes comes threaded all the way on from the manufacturer occasionally. Wrap a towel around the end piece(about 1/4 in) and clamp down on it while twisting the wrench fitting on the bottom(closest end to the Gas In QD) to loosen it. Then finger tighten or loosen to desired pressure.