Bending to the flow as the fin does allows the board to roll more easily, which is nice because the tunnel doesn't resist roll at all, and the two fins together offer only a gentle resistance to the board rolling. As the fin bends it stores energy and releases it as flex fins do, so it also helps the board spring back up out of the turn.
The feeling of the flex fin plus tunnel compared with the spitfire fin plus tunnel is a soft slightly twangy response to the rail to rail roll, the tail snakes through the turns, but doesn't crab sideways at all it still sets a true line. It's quite hard to explain. Smooth is a one word description.
Flex fins on their own can lack drive but the big tunnel is so powerful that the board has both drive and a soft forgiving response.
Here's a video of a few waves on the prototype, some with a flex fin plus tunnel ( stiffer flex than the new baron fin ) and some with the original spitfire fin plus tunnel . .. on some of the waves I'm hanging way back and using the tunnel more and on others I'm a bit further forward. I had to stall for the kneelo by the way so that I didn't run him over.
