for standard pool hoses, to prevent the pump from ruining hoses if it starts up the morning after they have been left attached. Suction alone holds these two pieces together, so as soon as the pump is turned off they will fall away from each other. Install the plug piece directly in your skimmer, and put the cap part on the end of your hose. After starting up the pump and setting to the appropriate power level, press together the parts.

Update: this worked perfectly for an entire pool season and is still intact, so I'd call that a success. No hoses lost this yeah to pump startups.