This year, the major contenders for the overall win at the Giro d'Italia, the elite stage race that began Saturday, are Italy's Ivan Basso, Levi Leipheimer of the U.S. and Italy's Damiano Cunego. All three are closely matched, setting the stage for one familiar difference maker in the Giro: The descent.

Of all feats in sports, few are as harrowing as this: Carving down a mountain at 60 miles per hour in a pack of 15-pound bicycles with tires as wide as a roll of Scotch tape, while wearing only a thin layer of lycra. Hitting...