This is an advanced form of speed control by increasing the pressure on one inside edge (for example the right ski), then releasing the pressure and shifting immediately to increasing the other inside edge (the left ski). Then repeat if necessary. Each increased pressure slows the speed. Alternating right and left allows the skis to remain parallel and point ahead without turning. The increase and release sequence results in the up and down motions of the upper body. Some skiers go down the top of moguls and control the speed by checking at the tops. This is how they can practically go straight down the fall line without gaining speed.
The snowplow turn is the simplest form of turning and is usually learned by beginners. To perform the snowplow turn one must be in the snowplow position while going down the ski slope. While doing this they apply more pressure to the inside of the opposite foot of which the direction they would like to turn. This type of turn allows the skier to keep a controlled speed and introduces the idea of turning across the fall line.