Most beginners stand up too tall when trying to turn their longboards.
Their body gets fully extended and then they fall back.
But the other problem is the total opposite. Some beginners can not turn their longboards because they are TOO COMPRESSED.
Surfer is trying to turn Left. She is very squatted down. Yes, here head is over her feet, but her butt is sticking way out. Look at the toes on BOTH her feet. She is leaning, not turning.
Here she is trying to Left, but her head is over the Right rail. She does "grab rail" to lift the outside rail and engage the inside rail to help her turn ( or maybe she was just too scared to let go her right hand ).
Here going back side right. Crouching down stance. Even though her shoulders are inline with the deck, the deck/board is leaning away from the wave. That is the reason why the outside rail is "catching an egde" and soon she will be flipped over on her face.
Compression is good and necessary to turn your longboard. His knees are bent, but also notice the twisting of his waist / upper torso.
Compression then leads to Extension. Action and Reaction.
There is a difference between coiling up and just crouching down.
Here she is not really crouching / squatted, but you can tell she is a bit scard / apprehensive in her turn.
There is a time and a place for crouching down and being very squatted. It is a useful tool to maneuver a longboard ( or any board ). But when you squat or crouch because you are afraid or trying to stay up right, that's when it is not good.
Look at the "good" surfers at your local surf break. Do they tend to be crouch or squatted ( and are stiff ) ?
Or do they mix compression with extension ( and are loose ) ?
Please check out Eason's surf photography and http://starb.on.coocan.jp/daily/daily0.html to see more.