Why a tucked-in spin speeds up
A spinning person has angular momentum, a measure of rotational motion. When outside forces exert little twisting effect, angular momentum stays nearly constant. Changing the body’s shape can therefore change how quickly it rotates without adding more angular momentum.
The key is moment of inertia: how widely a body’s mass is spread around its axis of rotation. Arms extended sideways put more mass far from the axis, where it takes more effort to rotate the body. Drawing them in brings that mass closer, reducing the moment of inertia. To keep the same angular momentum, the body’s rotation rate must rise.
The tradeoff behind the speed
Pulling the arms inward does not create spin from nothing. The skater’s muscles do work to move the arms, and that effort changes the body’s distribution of mass. The resulting faster rotation is the visible consequence of that reshaping while angular momentum remains approximately conserved.
The same principle works in reverse: extending the arms increases the moment of inertia, so the spin slows. A skater can use this change to control a rotation, but the effect depends on the arms moving relative to the body; simply holding a tucked pose does not keep making the spin faster.
That is why a small change in posture can produce a striking change in speed: mass moved closer to the axis has a much smaller rotational lever to work with.
