ARREAU, France — In this tiny commune in the Pyrenees, granite massifs and blue-green forests rise sheer, and we stand, myself and a crowd of several hundred, munching baguettes and sipping wine. We hear a great shout and then glance up the steep road to our east, where hurtling around the corner is …

… an aquamarine-clad human projectile.

Pello Bilbao of Team Astana is hugging the top cylinder of his bike, his butt off his seat, his head and upper chest extended over the front bars, looking like a tree sloth on a favorite branch. This human missile is traveling 45 miles per hour on this long descent in the Tour de France.

As just like that, he is gone.

How much control of the bike, I later asked Bilbao, did you have when you were tucked down like that? He shrugged.

“Not much,” he said. “I try to pray.”

Vaya con dios and all that.

Victory in the Tour de France often turns on minutes, seconds even, over a three-week, roughly 2,200-mile race — this year from the English Channel to the Mediterranean and back to Paris. In those weary days of racing across cobblestone and gravel and through humid river valleys, no test looms as quite so intimidating and grand as the mountain chains, the Pyrenees at the end of the second week and the Alps at the end of the third.