This is used so you don't have to elevate your kettle above your fermenting carboy after boil.
To explain how it works: 1 small hose from kettle outlet into carboy thru plug. 1 small hose from carboy plug to WW side. Hose from chiller outlet to WW. After wort is chilled, you put your chiller outlet on the WW and the flow of water causes a slight vacuum in the carboy,thru the hose, causing the wort to flow out of(or be sucked out of) your kettle into the carboy. No water in your wort unless maybe you get a problem and have backflow. Does seem like you would use alot of water to do this. It would be cool to figure out how to use compressed air to do the same thing.