Patented a few years ago by Hobie! https://www.google.com/patents/US2014013...brACokQ6AEIHDAA
The fins reverse for reversing propulsion and fold up/back on impact to reduce damage when striking underwater obstructions. Read the full abstract. I can't tell if the socket might be similar if not the same.
It appears there is a short lever at the base of the drive that you lift or pull to reverse thrust.
I don't really find myself wishing I had a reverse often, but it would be nice to totally stash the paddle for emergencies ..