How The Racers Ski, by Warren Witherell, published in 1972 is a book full of very modern ideas, still. It is amazing that poor ski mechanics like rotation and wedges are still debated today, 40 years after this book.
Quite literally, in the author's words, the entire book can be reduced to one sentence: "Racers carve turns rather than skid them". The central idea, exposed early on is that performance skiing happens with the skis and NOT with the upper body. Specifically, when the skis carve the snow due to edging and applying pressure.
He makes a big deal of just focusing on getting the skis to do what is needed and letting everything else fall into place. "Just edge your skis, relax and let angulation happen naturally". You focus on getting the skis at the right angle and apply the proper pression and leverage (fore/aft) and the upper body will naturally find a way to balance things out.
The general movement pattern, is to put the skis on edge and apply pressure to the front of the ski to start the turn and push the skis forward, to keep them moving and bring them to "neutral" to finish the turn.
A very useful drill to experience and commit to carved turns on one ski is to do carved turns with the skis wide apart... get the skis wide apart and edge/pressure the outside ski.
I his opinion, the upper body should be fairly erect and not excessively bent at the hip, with the hands forward. The role of the upper body is to maintain balance, given what the skis are doing, via inclination/angulation and generally staying relaxed and ready to react or anticipate.
Hips should be over the heels and he advocates a "square" stance during the traverses, with minimal rotation or countering.
A lot is spent discussing the need for canting and shimming, in this case via shims/wedges, placed preferably under the bindings. The canting is important for all skiers as it allows them to use the skis properly.
The forward lean angle of the boots can also be affected by shims under the rear bindings and it also is an important element of equipment setup.
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