I was pointed to this thread in another one where we discussed similar topics. I just wanted to add a small detail: the type of the bending discussed here is not concave (which would be good), it is convex (which is bad for the propulsion).
As I explained in the
other thread, and as already mentioned earlier in this thread too, the concave bending (ear bends) pushes water sideways instead of backward, hence a lot of energy gets wasted.
In contrary, at a concave flexing, the resulting bend creates a cavity preventing water from escaping sideways, and pushing so more water backward than a flat blade, hence it would improve the propulsion. But again, that would be the case only if it did not prevent the blade from flexing lengthwise (like in the photos with paper sheets earlier in this thread). That can be achieved only with a good design of the fin: reinforced sides (or arms or wings if you prefer), and a softer, thinner and possibly stretchable center or even a slit. Look at the following photo to see how the resulting concave flex should look like to improve the propulsion against the plain flat flexing common at current monofins (except the ones discussed here, that bend in the wrong way):
Atomic_SplitFin_Waterflow.jpg
Of course, the slit does not need to be left empty. On my mind, much better than a simple slit, is filling it with a stretchy material (profiled rubber foil) just like some bi-fin manufacturers do - for example Mares at the Volo Race fin (but there are some others too). At every kick, it creates a very efficient concave shape with a better efficiency than a flat blade, where much more energy is being lost laterally.
MaresFinsVoloRace410313.jpg
The same principle would apply at a monofin, of course too. You just need to find the right material, geometry, and stiffens (which needs to be variable across and along the blade).
