The action of your muscles against each other, and the connection of your feet to the surface you're balanced on. The applicability is the less mass you have moving at a given moment, the less mass you have to stop moving to go the other way, and the shorter the lag to changing the direction of travel of your CoM.
The biomechanics/anatomy of the motion is that the femoral head rotates in the acetabulum (hip socket). How you make that motion happen (what muscles you fire), and what amount of anchoring you have to the surface you're standing on determines what actually moves.
This can produce two kinds of motion:
- Rotation of the acetabulum around the femoral head (AF movement); i.e. Stand on one leg and move your body over the leg you're balanced over/standing on.
- Rotation of the femur in the acetabulum (FA movement); i.e. Stand on one leg and rotate the leg that's in the air
So when I (and PSIA generally) talk about rotating the legs under a stable pelvis - they're talking about using FA movement (the leg rotates inside the hip socket to cause the leg to move and the upper body to "remain still").
"Upper body rotation" (A movement of some/all of the torso from the pelvis up to cause a rotational moment around roughly the Z axis) is caused by AF motion, or spinal twisting.
SUPER PHYSICS NERD STUFF THAT IS NOT RELEVANT AT ALL INBOUND - PLEASE SKIP:
Higgs boson
"According to the Standard Model, a field of the necessary kind (the
Higgs field) exists throughout space and breaks certain symmetry laws of the
electroweak interaction.
[e] Via the Higgs mechanism, this field causes the gauge bosons of the weak force to be massive at all temperatures below an extreme high value. When the weak force bosons acquire mass, this affects their range, which becomes very small.
[f] Furthermore, it was later realised that the same field would also explain, in a different way, why other fundamental constituents of matter (including
electrons and
quarks) have mass."
And since Skiing is all about moving mass to fun places by interacting with a gravity well...