If you go to any google image search and search for any top surfer surfing you'll see what I mean when you look at their default stance when not in the middle of a big move. The back knee always leans in towards the front knee(very much the opposite of the poo stance you see in learner.)
In regards to hands, I meant arms. The shoulders and hips are never directly parallel with the line of the board. There is always one arm on either side of the body and board side to side(obviously trailing arm is quite far back).
When I practiced this on land propping the board up on a few towels to mimic the rail to rail movements of a board - this open stance is so much more stable than if your body is directly parallel to the line of the board. You can lean back, forward, side to side much further, and if you become unstable, squaring the shoulders up in an exaggerated fashion seems to stabilise you from falling off. I haven't analysed the last bit on surfing footage of surfers who land tough airs etc, but I'm placing a fair guess that is how they land and stabilise.