This is the infamous Ric Reiter demonstrating the infamous pivot slips!
This exercise is a series of linked 180 degree side slips down the fall line. To do these find some nice groomed packed snow with a consistant pitch and straight fall line (the line a ball would take if you rolled it down the hill). Begin with your skis across the fall line in a shoulder width stance and your hips and shoulders turned as far as possible to face down the slope. Then by simply releasing the edge grip with the ankles, begin to side slip. Practice this movement in both directions until you can sideslip smoothly down the fall line keeping the skis across the hill. Then, begin to link these by turning the feet beneath a quiet and stable upper body. Try focusing on turning only your feet and having your femurs rotate in the hip sockets. This mechanism is called "fulcrum" turning or turning one leg against the other and is a very powerful turning power. To make this turning mechanism work, you must maintain weight on both feet and keep a wider stance. Should you lift one foot or use a narrow stance you will lose the ability to use the fulcrum turning mechanism.
Practice this until you can do it well and your turns will improve.
Another common, but less efficient, turning power is the "rotary push-off" turn which involves sequential movements of the feet like stemming, stepping, or upper body rotation type movements which begin from a platformed ski which imparts a rotation down to the skis.
Good functional skiing avoids the rotary push-off mechanism in favor of the fulcrum turning mechanism. This means the are legs turning back and forth under a stable upper body. From here we can add in more edging or tipping as needed to shape the turns. So anytime you are making turns tighter than the design radius your skis can make you want to use Fulcrum mechanism to blend in the appropriate amount of rotary needed rather than stemming or involving your upper body to twist the skis.
Hope this helps.