Aren't those the crasiest-a$$ fins you've ever seen? They ride like crap behind the boat, but it was great fun to test them.
Anyway, the little rail channels aid squirt and seem to lock the board down a bit. If you've ever looked at the spray coming off your board as you're riding, there is a BUNCH of water that gets pushed off to the sides and these little channels capture some of that and send it out the back.
I didn't get a good picture, but both of these boards have a single concave and the channels are cut into that concave.