What is happening in that top one? I can't quite work it out.

Thats interesting footage I too can't work out what is happening with the top one. I was very surprised to see how much the fin flexes under water, the box has to bear quite a load.

The top one is a sculling oar, not a windsurfing fin. If you look at some of the other videos after it finishes, there's one of the lady actually sculling the boat. She's going surprisingly fast for the amount of effort she's putting in.