This is attached to the gas port of a keg (Cornelius or Sankey or ...) which has been precharged with CO2. Keg liquid in port is connected to fermenter with valves closed. Needle valve on device is opened to bleed keg pressure down to just below fermenter pressure. Liquid valve is then opened and beer flows slowly into keg while pressure is monitored on gauge. Needle valve is adjusted for proper flow rate.