It’s not possible to show how much flex occurs when riding waves. This shows a little. I’m almost 200 lbs in these pics as I’m standing on the bottom of the hull. The forces at work when riding with me kneeling on it are considerably greater!

You may be able to tell that the width of the board changes, the overall rocker changes, the bottom rocker changes, the shape itself changes, everything changes. And all that from flat planing. Weighting and unweighting add considerably to the effects as do moving my center of gravity around.

Can you see the rocker change?

Turning introduces a whole new set of dynamics.
Board twisting and bending, tail(s) flexing and lots more.

Someone is standing on the tail of the board where the fin goes so the tail is remaining flat on the ground.