Joe, the trick in 'freeze protecting' the MD is to blow air through the line until you see bubbles, then slam the line shut. It isn't the best way to protect the line, but (unless you care to don SCUBA gear) it's the 'traditional' way of doing it. What you're looking to do is getting the water in that pipe below the 'frost- line' so that the water in it won't freeze and burst the pipe. This is one of the reasons that some builders will not install a MD on their pools.