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. 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. Gauge reads 0 - 15 psi