I came across this page again, and there's some similar biomechanics:

https://surfstrengthcoach.com/the-perfect-surf-stance-to-improve-your-surfing/#

The Perfect Surf Stance:

A dynamic posture allowing for fluid movement due to efficient skeletal positioning and maintenance of the body’s center of gravity over a moving and fluid surface. Optimal alignment of joints to permit rotational ranges of motion, and allowing a spring-like action to occur through the joint centers of the lower body. A balanced position creating even weight distribution with allowance for pressure differentials to influence surfboard rails based on the movement of the upper body.

I just made that up, but it sounds fairly spot on. Bio-mechanically it is 100% spot on and correct, but to you, it probably makes absolutely no sense at all, which is fine, it’s a lot of professional jargon really that I would say to impress other surf coaches.

Here are the basics of the surf stance: Stand in a spring-like forward-facing stance so you can easily move in all directions, up, down, and rotationally.

It’s an athletic stance, almost like a boxer being light on their toes, and springing through the hips and knees. At its lowest point, full compression, think of a tiny little barrel crouch, it is what I refer to as a sprinter position. That sprinter in the blocks ready to run position, capable of exploding forward and producing momentum. At its highest point, extension, it’s that stacked boxer / fighter stance, capable of being a spring.

Don't be a Poo man/woman:

So please, for the love of good surfing, stop throwing your back arm backwards and flailing it around.