This product is used to relieve pressure when transferring or filtering beer from one keg to another. This connects to the keg you are filtering into and has adjustable relief valve for letting pressure off as the keg fills up.
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.