Some pocket knives are pretty wicked.. a guy around here was attacked by a cougar last year, and he managed to get out his pocket knife as it was chewing on his arm and cut it's throat, killing it. As I recall it was an expensive, high tech knife with super-sharp blade... deadly.

As for the hiker's blood loss, after 5 days blood flow was likely pretty much shut down to his arm. Presumably he was able to get a tourniquet above the location. I haven't read where he cut, I guess at the elbow would be easiest to get through but some knives have a saw back that could cut through bone.

What I don't get is how he rapelled down a cliff on his way out - most people couldn't do it whole!

A incredibly tough and determined guy.