Comprex, IMHO, the ultimate reason for falls like the one you described is that the skier is so used to skidding his turns that he manages to stay upright on the icy patch with his skis pointed in some other direction than the direction his CM is going. However, as soon as he hits the patch of thick stuff from the guns, all sideways motion of his skis is stopped immediately, but his CM keeps going, and so he falls headfirst to the downhill side of his skis.
The solution is obviously to always have your skis pointed in the direction that your CM is going. This is just another way of saying, "carve", or "ski the slow line fast". Obviously, this approach isn't going to be possible for everyone, but it points out why one should work towards what the pros consider as "good skiing", even if done at very modest speeds on easy terrain.
I must confess that because of the icy <-> wet&soft <-> icy <-> wet&soft variable conditions, I was a bit worried about the three level 3 high school girls I had in my charge the same night as the accident I described earlier. I made it my business to be sure I set a good line and modest speeds so that even with a shallow gliding wedge, their skis never were at a large angle to the direction they were going. I was very happy to see that they would go right over these patches and into the thick stuff without so much as a bobble, while other people were falling all over the place. I know they didn't even realize that what they were doing was a big deal, so I had them watch some classic skidders and observe the difference.
I know the above didn't directly address your question, but I think its important to always understand what the "real fix" is in addition to describing "what-do-you-do-after-it-happens" suggestions.
With respect to the latter, there actually has been discussion on Epic of how to manage a slide, but nothing recently, and the older discussions often concentrated on the problems of higher speed tumbles on steep exposed terrain. IMHO, managing a slow speed slide on anything but a double black at a ski area is pretty do-able.
Specifically, you do exactly what they teach tourists in mountaineering schools to do when they are introducing them to the use of the ice ax - you concentrate the pressure that you are exerting on the snow in the smallest possible area. The way to do this is to flop over onto your belly (if you aren't already there), un-twist your legs and skis (if this is a problem), and finally, you do something like a push-up onto your hands/forearms, and your ski edges (or boot toes, if you have come out of your bindings). If you were using an ice ax, you would first have to be sure to spin around to be going feet-first, but on skis at low speeds, this is less important, IMHO. If you *are* oriented feet-first, be sure to keep your knees flexed to absorb the shock as your edges (or boot toes) periodically dig in and release their grip.
Usually, this procedure will bring you to a quick stop. The reason for the repetition of the "slow speed" caveat is that once you are going above say 10 mph, if your edges suddenly grab, you are at risk of injuring your legs/knees, or going into a high speed cartwheel and loosing complete control of your descent. If you are sliding at higher speeds, my recommendation would be to first scrub off some speed on your forearms and knees (assuming no rocks), or using a pole tip to dig in and self-arrest, and then start to carefully engage your ski edges or boot toes.
Tom / PM
[ January 09, 2004, 12:53 PM: Message edited by: PhysicsMan ]