@Bad Bob Had the right short answer....Dynamic Balance. Balance while on the move.
But as
@T-Square pointed out there is dynamic balance and then there is dynamic balance on a moving platform.
Off skis, we humans dynamically balance during the creation of locomotion (walking running skipping etc). But this method of dynamic balance is totally different from the method required for skiing. Human locomotion is accomplished by pushing and catching our mass using the opposite ends of our feet in a "rock over" method . We rely heavily on plantar flexion to push and absorb. On skis we need to learn to consistently and reliably balance through the arch supported by the balls of the foot and the heel but making sure that both support points are secure on the boot bed. How much we can move fore and aft under this construct is called the "Cone of Stability" See Kipp's video below.
@Steve has it right with articulation as the way we achieve dynamically balance through the arch. The ankles are the most important as they create the boot shin interface. Contrary to conventional wisdom the knees are relatively inconsequential as they are an intermediary to the flexing and extending of the ankles and hips. The hips are the most critical because in skiing, they don't tilt forward as they do when creating locomotion. This is a subtle but important difference.
Ron Kipp's "Flex Your Ankles" Video provides us with the fundamental magic move....Dorsiflexion..... the dynamic tension it brings to the ability to maintain a centered stance from which all the other "magic moves" emanate. .
Stand up and begin bouncing up and down (using the 3 hinges) so that your COM goes through the arch. Focus on the hip rotation. Pretty easy to do but you can see how quickly you could loose that balance point if your were on ice. Now, create tension by flexing your mid section (core) and dorsiflexion at the same time. You will see how the tension applied on the top of the hinge system and on the bottom provides a very controllable, stable platform bounded by the balls of the feet and heel.
It is the tension from dorsiflexion (vs plantar flexion) that is new to dynamic balance on a moving platform equation. Once a skier gets command of this movement pattern all the other "magic moves" will be possible.