Parallel turn
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The parallel turn in alpine skiing is a method for turning which rolls the ski onto one edge, allowing it to bend into an arc. Thus bent, the ski follows the turn without sliding. It contrasts with earlier techniques such as the stem Christie, which slides the ski outward from the body ("stemming") to generate sideways force. Parallel turns generate much less friction and are more efficient both in maintaining speed and minimizing skier effort.
Parallel turns require solid contact from the skier's lower leg to the ski to rotate it on-edge. This was difficult to achieve with early ski equipment, limiting the technique to the high performance realm of racing. The introduction of composite skis, metal edges, releasable clamping bindings, and stiff plastic boots combined to allow parallel turns even on beginner equipment. By the late 1960s it rapidly replaced stemming for all but very short-radius turns. The evolution of shaped skis in the 1990s advanced the carving turn to preeminence.
Today parallel turns are taught to teach novice skiers the effect of weighting and unweighting their skis.
Basic action
The parallel turn relies on two dynamics: turning the ski up on its edge and bending it at its center to create an arc.
The skier initiates the turn by moving their knees laterally in the direction of the desired turn. The motion of knees is translated through the calf to the high cuffs on the ski boots, through the boots to the bindings, and then to the skis. This causes the skis to rotate up on their edges, with the skier's weight and the force they put upon them bending the skis at their center into the arc that turns them. To stop the turn the knee is rotated back to the neutral position. While both skis take part, in practice the ski on the outside of the turn is dominant.
Moving the legs to the side shifts the center of gravity, compensated for by moving the hips in the opposite direction. The effect is to keep the skier's upper body upright while the lower torso and legs shift side-to-side. The skier pressures the front of their skis to keep the fore-and-aft center of gravity of the skier's mass over their toes.
The parallel turn can be improved through dynamic "weighting". Turns are often linked in a series of continual arcs, one direction then the other. Lifting the body through the middle of the switch partially releases the skis arcs, easing the transition to the opposite direction.
Changing technique
Traditionally construction techniques produced skis with limited torsional strength. The minimal sidecut this permitted resulted in parallel turns of limited radius. Consequently, paralleling and stemming were mixed together depending on the situation. Often turns would be initiated parallel then evolve into stemming to create greater turning power - at the expense of both speed and increased effort.
In the 1990s skis were progressively widened at the tips and tails relative to the waist. Applying an edge of these "shaped" or "parabolic" skis brings a curved surface to the snow, resulting in a carve turn.
References
[1] http://www.youcanski.com/en/coaching/inside_ski.htm
[2] http://www.mechanicsofsport.com/skiing/manoeuvres/parallel_turn.html
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