WHAT DOES THIS DO? When violent fermentation in your primary has slowed, transfer your juice to the secondary by pouring through a funnel, leaving most of the sludge behind. You could also pour through a strainer bag. It helps to have a second person hold the funnel or bag steady. Then all you do is fill the airlock with water to between the lines and drop in a few specks of metabisulfate powder. Put the cap back on it and then just push it straight down 1/2″ to 3/4″ into the 3/8″ hole in a stopper, cap, cork, grommet or bung sized to fit your secondary. CO2 pressure will build and go up through the pipe and force the float upwards until it bumps the cap. In a few minutes the excess gas will bubble out around the bottom of the float with an entertaining bloooop! Get several going at once for a happy little concert. When the bubbles stop, fermentation has come nearly to an end.