Yet it's safe to say it was a shock to almost everyone when Voeckler not only held on to his lead over all other contendors for the yellow, but was a primary activist in chasing down attacks. I'd have expected Voeckler, by any reasonable measure outmatched in that group, to take a very conservative approach to the climb, following the smoothest wheel to avoid going into the red for even a few seconds. Cadel Evans and Alberto Contador are both exceptional in their ability to put on short bursts of amazing climbing speed. So if Andy Schleck attacked, Voeckler could have let one of these riders close the gap, perhaps following Basso who would steadily bring it back together afterwards. But instead Voeckler was sprinting back to Andy's wheel himself.
Voeckler's special motivation on the climb.
Meanwhile, Andy was tempering his attacks based on what was evident weakness in his brother. Fränk obviously wasn't feeling as frisky, and the risk was an attack by Andy could result in nothing more than Fränk getting dropped from the group. That would hardly serve his team's interest. Of course, best of all would be for Andy to attack and get away solo, something a lot of his fans obviously wanted to see, but Andy had a good view of how his brother and the others appeared, and if Fränk was a weak link, he may have realized the team would do better by staying united at least until the upcoming Alpine climbs.
All of this start-and-stop worked to Voeckler's favor. Voeckler is also a punchy rider with a strong top-end, but where he'd have most likely succumbed would have been a faster, steady pace. The lack of cooperation among the favorites also allowed first Jelle Vanendert and later Samuel Sanchez to ride away from the group -- Jelle is out of overall contention and Samuel, although now 6th overall, is a poor time trialist and so still needs to gain considerable time in the mountains to be considered a threat. So by cooperating the favorites obviously could have climbed faster, likely faster than Voeckler could have sustained. But dropping Voeckler wasn't the priority, since Voeckler also is considered weak in the time trial, and so I don't believe the favorites consider him a likely threat to still lead in Paris.
Science of Sport plot of great VAMs in Tour history. Voeckler's 1600 on Plateau de Beille is well off the chart.
This is all deja-vu for Voeckler, who also rode to defend his yellow jersey seven years ago on the same climb. He finished that stage 13th, 4:42 behind Lance Armstrong and Ivan Basso, to just hold on to his yellow.
So what is it that allowed Voeckler to go from a rider who got dumped by close to 5 minutes by the favorites to one who was in the mix, matching their accelerations to the end?
Michele Ferrari named and popularized the use of the VAM metric: the rate of altitude gain of a rider on a climb, typically measured in meters of altitude gained per hour. It's a decent way to objectively compare rider power/mass ratios, especially when comparing climbs of a similar grade and altitude difference.
The results: Vanendert finished the climb in 46:01, a VAM of 1627 m/h. Samuel Sanchez was 20 seconds slower, then the favorites finished Samuel Sanchez was not far behind (20") and the group of favorites did it at 1600m/h (5.75 w/kg), in 46:48.
In contrast, according to Wikipedia Lance Armstrong reportedly did the climb in 45:30 in 2004, and Marco Pantani climbed it in a remarkable 43:30 in 1998. In 2004, Voeckler finished 4:42 behind Armstrong, making his time 50:12.
So it appears Voeckler was able to take 3:24 out of his time from seven years ago. But the stage in 2004 was 206 km ridden at 33.8 kph, with seven rated climbs:
In contrast, this year's stage, while still extremely tough, was clearly easier: 168 km with six rated climbs (CyclingNews) ridden at 32.2 kph. That's 1.6 kph slower than the 2004 stage, which had an extra category 3 climb. So it's reasonable Voeckler would be faster this time.
It's no surprise the speed was higher in 2004. The finishing list looks likes a who's-whom in mid-naughts doping scandles. Of the twelve riders finishing ahead of Voeckler only the last, 12th-place Stéphane Goubert, has not been involved in a major doping scandal. I think it's safe at this point to include the stage winner in the "involved" category, given the consistency and number and specificity of charges against him. So it's really remarkable Voeckler was able to hang with that crew as long as he was without totally collapsing on that final beyond-category climb.
Anyway, Voeckler's weakness is his time trial, and thus I think he has little chance to win the yellow when the Tour arrives in Paris. And while a VAM of 1600 was enough on the tactical shadow-boxing on Plateau de Beille, it's unlikely he's going to be able to follow a more sustained attack during the back-to-back big stages in the Alpes.
added : Cyclocosm has an excellent comparison of times, which differ slightly from Ferrari's (it depends on if you time the group or the individual riders, since it takes a significant time for the pack to cross the threshold of the climb).