I found this on VeloNews, in the Tyler Hamilton Giro
Diary. It details what happened in stage 5.
"I had a mechanical failure in my rear wheel and I found myself sailing toward the ground like a missile. I was literally thinking to myself "this can't be happening" as I was diving toward the pavement.
It turns out the pins that connected the freewheel to the hub of my rear wheel came apart. The effect of having something like this happen while gunning down a decent is very similar to breaking a chain mid-sprint. As I stood up to pick up speed going through a turn the cassette broke free from the wheel. I basically launched myself over the bars and into the air. If Carlos Sastre hadn't been there to give me his bike after the crash, I probably wouldn't have seen first group again. "
If you want to read the whole thing it can be found
here
Marty