My guess is that it's part of a kit for climbing your mast. You put a taught rope through the cam cleat, stand in it then draw it up with your feet while taking your weight on another cam cleat or ascender attached to your climbing harness and / or bosun's chair.
I'd take a guess at some form of mast climber - pull your feet up for going up, and squeeze the 'antenna' to release on the way down - assuming you're in a bosuns chair!