That is fins designed to provide lift when cavitating, like cavitating propellors and the cavitating foil on Sail rocket.
They are inefficient at slow speeds (I'm told) but thrive at cavitation speeds like 50 knots.
Is there such a beast in windsurfing?

Last time it was talked about, my memory says, the conclusion was that they are too draggy to get up to cavitation speed on a windsurfer.
But that was a while ago, things may have changed, as they do!