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When I launch off a feature skiing, I tend to throw my arms backwards to try to stop my forward rotation. This is quite common, and it feels like it works. However, seeing as my arms and body are a closed system, how could this possibly do anything?

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    $\begingroup$ What do you think happens to your centre of gravity? $\endgroup$ – QuIcKmAtHs Feb 3 '18 at 3:28
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Throwing your arms back will throw your upper torso forwards, increasing the forward rotation. If this does "work" it is probably that by spreading your arms out (not throwing them back) you increase the air resistance on your upper torso, which rotates your body in the backwards direction.

Rotating your arms in the same direction as your torso is going (ie forward rotation, your arms going down and back) will also counter the rotation of your body. See Can you remain balanced if your CG passes in front of your toes?

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The skier is trying to maintain his balance. One way to illustrate how the centre of gravity is prominent here is the following situation. Standing against a wall, one cannot grab a ball which is considerable amount of distance in front of him, without bending his knees. However when there is no wall, he can kick out his leg(which is still straight) and grab the ball without falling over. This sticking out of the leg is to ensure that the centre of gravity is along his stationary leg. Back to your scenario, the skier attempts to do the same by throwing his hands backwards.

In addition, throwing your arms backwards can be seen as a counter to your torque when you spin. This also hence allows you to stop spinning, and come to a stop.

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