Initiation Phase
This is the preparation phase of the turn. In racing, the timing of initiation in terms of line is imperative. Too soon or too late each create their own problems; both result in related technical problems and ultimately slower times on the racecourse. Ideally, turns will be started at the "rise line" to insure maximum speed from the turn. The pole plant will help to mark the start of the turn.
Balance should be on the ball of the foot to properly initiate the turn from the front

of the ski. The feet will be placed outside the body in an angulated fashion to create edging and proper dynamic balance to the dominant ski(s). The legs and body may be somewhat extended, while keeping proper ankle and knee flex and balance directed toward the front of the ski(s).
If the turn (arc) to be made is tighter than the skis can carve cleanly, the skis may need to be redirected (steered) towards the new intended direction. If steering is necessary, all efforts must be made to reduce friction during this steering, while allowing the athlete to prepare to carve cleanly through the remainder of the turn. To optimize racing speeds it is fundamentally important that the athlete carves cleanly in the final phases of the turn to maintain speed and momentum.

Controlling Phase
As the name describes, the majority of radius of the turn is controlled during this phase. Early pressure and good body position to the dominant ski(s) will bring the skis into the fall line with a clean carving action.

Completion Phase
This may be the most difficult portion of the turn and may require the most attention and practice. It is imperative that the skis exit the turn carving cleanly in order to maximize the skier's momentum and speed into the next turn. Understanding that the exit of one turn sets the tone for the initiation of the next belies the importance to the completion phase of each turn.
As most turns require a connection and flowing movement of one turn to the next, the completion and preparation of turns are often very closely linked. Regularly, the athlete will be preparing to start the new turn while still completing the last turn. 

by SvetSki