The invention of the Parallel turn in skiing is credited to Austrian Toni Seelos in the 1930s.
The technique involves setting the uphill ski so it runs along its inside edge, and applying pressure to it. Because of the way skis are shaped (narrower under the foot) the ski will turn (to the left if the pressure is to the right ski, to the right if the pressure is on the left). As the turn progresses the two skis remain parallel, and the second ski also starts to run along the inner edges.
The turn can only be made if travelling at some speed, since the speed of the skier generates the pressure needed to turn the skis properly as the skier banks into the turn.