From my experience, it would take a brave soul to remove a valve with this tool, or any other. I can't recall ever removing a valve that didn't require someone on the outside with a proper thru hull tool to hold it, or turn it while someone held the valve.
Strikes me as a Ronco $9.95 tool that if you call within the next ten minutes, they'll send you TWO worthless thingys to clutter up the boat;
when a wooden plug will work as well..
Who knows though. I'm just spouting contempt prior investigation..