Eric S. Raymond
I have trained in knife, empty hand, and knife disarms. What @ThresherKing says here is entirely correct - you should not even attempt a disarm unless you have no other options. If you have a firearm, you should keep shooting until you see the knives fall out of the perp's hands.
The Thresher King
The comments are rife with morons who, having the benefit of watching this on a 4" screen, think they would have done a better job than people trained to deal with these scenarios. I was trained to disarm knife-bearing attackers. Some of the knife bearing attackers I trained