These are screen shots from today's stage 7 of the TdF. There was a gradual descent about 11.7 km from the finish, which afforded opportunity to watch the descending technique of the leaders of the peloton.
This is Sylvain Chavanel in a near textbook application of Obree's preying mantis, sans aero bars, except his elbows are too splayed. Chavanel is so far forward in this photo he very nearly has his nipples on the handlebars.
The Maillot Jaune in a slightly less radical interpretation but still distinctly mantis-ish. Nibali's chin is about even with the bars.
In either case, ideally, the crank arms should be horizontal. Chavanel's pedals are slightly out of position because he is riding knock-kneed, attempting to clasp the top tube betwixt his thighs to stabilise his "off the saddle" position.
The only real variations I saw to this technique in any great measure was how far forward the rider moved, and where the hands ended up. To bring the head forward of the handlbars, one's tush almost has to be off the saddle, which shifts some load to the legs. So in addition to the potential instability caused by the radical weight shift, some might also prefer to keep bum on saddle and give the legs a short respite in the descent. And regardless of their head position, everyone's hands tend to come together on the handlebars, at least to no more than shoulder width, when it doesn't unnerve the rider to have his hands so far from the brake levers. And elbows should be tucked.
Watch the video for yourself, paying special attention to the descent just inside 12 km, and decide for yourself.
Of course you can't descend in that position if there are alot of turns.
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