Action and reaction is a fundamental principle of physics, playing a big role in skiing, in the movements that we do.
The basic principle is that for every action, there is an equal and opposite reaction. So, when we press against the ground with our feet, the ground will push back with an equal force.
Read some scholarly articles, if you need some highschool physics refreshers:
When the skis "dig in" and create a high speed carved turn, the ground pushes back against our inertia, creating a "centripetal" force which bends the skis and creates the round trajectory of the turn - all modern skis have design elements to use this effect, so they for instance bend in a tight or larger arc as needed, making use of more or less edge. Some are stiff and cambered for hard snow while others are soft and rockered for soft snow etc.
When we push into the skis on edge, the ground will push back, with a force distribution along the ski according to the contact (sidecut shape) and flex pattern of the ski, resulting in the ski bending into an arc:
When we move the hips forward and press against the ground more forward, the ground reacts and bends the tips of the skis more, resulting in a tighter turn.
When we counter-rotate the upper body, there is an equal rotation of the lower body. If the boots were anchored by the skis on edge, there will be an equal reaction from the ground, pushing the tips.