Hello Doolio, thats an excellent question - and the subjects touched on there are often debated among coaches/instructors. I think that you'll find good input from many other members than just Bob and I on this. Here are my initial thoughts on this:
>>How would you explain, to a student, why the main point of foot pressure changes from toe to "arch" to heel thoughout the arc of a purely carved turn (or from front to back foot for boarders)?<<
First I would say that in my experience its not a given that there *is* always a pressure change. You can stand neutrally (in the center) of the modern skis and carve a clean turn. However, to change the radius/shape of a turn you can choose to change where you are weighting your ski. Now there is a difference between simply shifting the weight under your foot - and pushing forward or pulling back against the boot cuff/shaft.
Weight distribution changes only controlled by direct pressure under the foot do make changes in turn shape, but they are more subtle than changes made by *leveraging* the tip or tail of a ski through use of the boot shaft.
It definately was a given on the older skis, unless you wanted to make a very long radius arc. To make shorter arcs one generally had to use selective leveraging of the ski through the turn.
Racing on the previous generation of skis I had my body well trained to initiate a turn on the tip of the ski(s), control the turn in the center, and finish on the tail. Then move your "center" (weight) aggresively forwards back to the tips of the skis. I found that for me it was most effective to think of "pushing" my feet forward through the whole turn - which resulted in a smooth rolling of the weight across the ski, and then to start a new turn 'falling' back diagonally across the fall-line to cross over to the other side of the skis and simultainously get my weight back on the tips of the skis to start the new turn.
When you watch top racers and free-skiers today you'll see that they spend a great deal of time in a more neutral position on the skis - unless they need to make an adjustment to their line. When we do need to make this adjustment we can shift our weight fore or aft or for more extreme changes leverage the boot forward or back. The new skis are much more responsive to this weight shift as well.
When I first tried the early prototypes of shaped skis - 10 years ago or so now, I remember that the skis seemed so reactive that they scared the hell out of me! Thats because I was still habituated to using a powerful leveraging to create the turn shape I wanted in a carved turn. The skis say "alright mofo' --- you want to turn that sharp . . . here!"
It would catch me off guard and I couldn't keep my center up with the skis, and was constantly getting left behind.
Anyways - sorry about the long tech stuff, looking back your question was actually how to explain it to a student. I think that you obviously don't need to go into the kind of detail I just did, and it would in fact be counterproductive (unless they ask for such detail - which some advanced students will). For me its worked best to have them experiment with the extremes of weight changing and leveraging. I.E. -- doing turns where they are totally neutral. Turns where they are all the way forward for the entire turn, all the way back for the entire turn - and then mixing it up in turns. Their bodies will figure out the result of weight distribution changes much more quickly than most folks will be able to apply it with a technical explaination.
>>Why do my Elan monoblock scx parabolics J-hook the completion phase of a purely carved turn? Is this true of most parabolics? This does not happen when I am carving on radially-cut shapes.<<
You, like me, probably cut your ski teeth on skis with much less sidecut. So you have trained your body/mind to weight and leverage with intensity and timing optimized for that equipment. I definately had to learn to be much more subtle on my skis when I moved to extreme shapes.
>>I've been reading ski reviews that only give a ski's tip, waist and tail dimensions and mention nothing of its carve radius. Is there a formula to do this that would also tell if the ski is parabolic or radially cut? The last part may be impossible I realize.<<
You will usually find the turn radius if you search hard enough - perhaps on a review site/magazine if the manufacturer isn't advertising it. It is generally referred to as an __X__ meter radius sidecut or something similar. For example some of the more radical sidcut skis can pure carve a round turn as low as 8-9 meters radius. And of course if one only needs to carve part of the turn (like you'll in many of the turns in SL racing) the turn radius can be even shorter.
Good subject, I'll look forward to hearing what the other Bears have to say on this. Thanks for helping me get my brain warmed up for ski season!