Utilization of rotary movement control
Simple crossunder involves shorter turns executed with the upper body aimed at a particular point down or across the fall line. This is one of the few instances where obvious counter-rotation is employed as a turning force, or as a means of controlling the dynamics of a solidly carved turn. This turn mode is relatively simple, and requires only that you rock the board from edge to edge, maintaining ‘correct’ orientation of the upper body and hips. You should avoid the tendency for the upper body to begin swinging, as this will defeat the effectiveness of counter-rotating. your hands should be held in front of the body, palms down, elbows out, in a position that is relatively comfortable to maintain. If your hands are not put in a ‘set’ position, they will tend to swing about, contributing to the undesireable movement of the upper body. If done correctly, you are simply rebounding from one sidecut to the other, with a set rhythm. Imagine that your hands are sliding downhill on a set of parallel greased rails.
Dynamic crossunder involves a shaping of the turns, both in depth and breadth, through an extension and retraction of the legs during the turn. There is a definitive movement away from a set rhythm, to a mode in which the shorter radius turn can be executed at will, not just when the rebound energy dictates. You should begin within the structure of the sidecut rebound turn (Simple Crossunder) on relatively flat terrain, gradually retracting your legs at the turn connection. This move seems to work best if done progressively, which is to say that the retraction/extension is not implemented at once, but grows in amplitude as you become familiar with the movement and its effect. To get the idea, imagine traversing a mogul field on skis, maintaining base contact with the snow. In order to accomplish this task, your legs must extend and retract to match the contour of the bumps. Take that same motion, and time the extension to match the apex of the turn, and the retraction to match the turn connection. With practice, you should be able to tailor the shape of the turn at will. Notice that at this point, the turn shape is still a clean, crescent shape, symmetrical both internally, and from toeside to heelside. (Even foot pressure, even angulation/edge angle).
On a sunny day, sometime around noon, find a liftline trail of relatively low pitch and consistent snow conditions. Ride in the fall line, using the shadows of chairs on the lift moving uphill as slalom gates. The movement of the shadows and your downhill speed will not likely work out so that you may turn with a consistent rhythm. For that reason, try to edge only enough to get the board to change direction, and no more. You are not trying to etch a continuous line, rather a staccato series of short, sharp parabolas. You should feel as though there is a moment of hesitation between each turn, a moment in which you may feel you have all the time in the world to execute your next turn. As this becomes easy, find a steeper liftline trail, or one with variable pitch. Additionally, you can use every second shadow for several turns, then every shadow for a few more, or any other combination which might occur to you. The main idea here is to develop adaptability to rhythm change.