It's called blow off. If you're using a 3-piece airlock, take off the cap and the inner bell-shaped piece and stick a piece of tubing onto that hole. Run the tubing into a container (1/2 gallon or larger) filled with water and sanitizer. If not, stick the tubing through your stopper or grommet. This is a blow off valve. It works the same as an airlock, but it allows for overflow.
Also, tridean, any chance you are, or were recently in Japan? It's a long shot, but I knew someone back there who might go by the name tridean.