From what I've read, the issue wasn't being able to breathe. It was cutting off blood flow to his brain. If you've ever seen someone put in a "rear naked choke" (they really should come up with a better name for that), that's what it does. It blocks the blood flow to the brain through the carotid arteries. It takes only a few seconds to make someone black out and could most definitely kill them if not released pretty quickly. I'm guessing the knee on the guys neck for several minutes did basically the same thing, albeit a little slower than the choke hold since it was only on one side.