I'm not sure what segment of the race you were watching, but I think it's not so much trying zig zag up a hill. Instead, they are going a bit faster than most of us and they are optimizing their line for the curves and turns in the road. That is what is going on in this long clip from Flemish TV: http://www.youtube.com/watch?v=yyeRfrAFS0Y (those are the last 6km of today's stage in the Vuelta). Over in the USA Challenge in Colorado, we saw a cat and mouse game, where on rider, Ten Dam (Rabobank), kept trying to escape with a quick acceleration moving away from the other 3 riders in the break.