Hmmmm. This may not be specific to your question, but I believe in general this may help.  It gets back to the perception vs. reality concept, i.e. when we pull across course, we perceive that we are going 90 deg to the course, when in reality we are going diagonally.
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What I think squaring (shoulders always facing downcourse) causes is to help use the hips and knees to move the ski and center of mass, rather than using the shoulders to initiate the movement.  Another way this can be described is that the upper body is still and the lower body is doing the work.
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A big key is to focus on simple things that make your body position more efficient or natural. Thinking about always keeping your shoulders facing downcourse, like a snowskier always having his upper body facing downhill, can key your body into a much more efficient position. That is one simple thing to focus on instead of thinking of arms in, knees bent, quick edge change, head level, etc, etc.  You want a simple key that makes other things fall naturally into place.  Focusing on “shoulders square†is one of those very effective simple keys.
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Now the reality part. If you look at the really good snowskiers, their upper body isn’t facing perfectly downhill, even though that’s their goal.  For waterskiers, it gets more difficult since we have one foot in front of the other, we will never get the upper body facing perfectly downcourse.  The key is that if we attempt, or feel, like the upper body is facing downcourse, you will end up with a much more effective body position.
Bottom line is to take the photos with a grain of salt and focus on what you are perceiving.