Originally Posted by mdf
You seem to be saying that how you hold your body will, in and of itself, change how it accelerates. Surely that is not what you meant?
Yep, in a way you said it correctly. But not exactly the wording I would prefer.
For a different perspective, consider this...
A person standing straight up on a rock has no reason to move forward - inertia keeps them right where they are. It doesn't matter if that person is standing stationary on a rock (zero speed) or standing on a Jet Aircraft going a constant 500 mph in a straight line. Nothing is 'pushing' this person's body forward or backward off their feet.
And it's not Level Motion (horizontal) that matters. A person can be standing upright on an Escalator (going up or down) and the situation doesn't change. We don't lean forward when going down an Escalator do we? Nope, we stand upright. Our upright body moves at the same speed as our feet, albeit somewhat downward.
The person who leans forward while standing on that rock will not only fall over - the speed of their fall will accelerate
at a rate that depends on the current angle of their body. The more off-vertical their angle; the higher the rate of 'falling' acceleration will become.
Now picture a person standing up and facing forward in the aisle of a bus.
When the bus accelerates forward the person's feet
are forcibly moved forward by contact with the floor. But nothing
is directly pulling or pushing the person's upper body
forward so they fall over backward. This person must do something to compensate for the acceleration of the bus floor - and deliberately 'falling forward' will do the trick. The person need only lean forward
to some compensatory degree.
The trick is to match the rate at which their upper-body is constantly Toppling-Over Forward
with the rate at which the bus floor is accelerating forward. When they match rates they are STILL continuously 'falling forward' but the Bus floor keeps pulling their feet up underneath them. By leaning forward they've balanced their upper-body's forward acceleration with that of their feet.
Getting back to skiing it's that same Relationship between individual Rates of Acceleration that's important.
If we hold our upper-body too far 'back' of our feet it will not be accelerating as fast as our feet - the rates will be "out of balance". If we stand at an angle too far 'forward' then our upper-body will be trying to Topple-Over Forward faster than our feet/skis can accelerate forward into a supportive location.
Getting back to your uncertain statement: "...how you hold your body will, in and of itself, change how it accelerates..." you now have some support for the idea.
Holding our upper-body at some angle off 'vertical' alone (wrt Gravity) will
in fact change the rate at which it accelerates (topples) forward. In skiing we manage our Fore/Aft balance by maintaining a 'compensatory' rate of upper-body-falling-forward vs. the rate at which our skis are accelerating forward.
The angle at which we hold our upper body does in fact control the rate at which it accelerates. If a skier feels like their upper-body is 'being pulled backward' it's the mis-match in rates of acceleration that is the culprit.