On a highly charged political case, I have reason to doubt everything any "experts" claim. The Minneapolis police training materials say this technique was allowed. If it were fatal to use this technique, I think it would have been known before George Floyd came along, and then it wouldn't be in the training materials.
Earlier today I read of a Freeper who had a friend do this to him to see if he could breath. He said he could breath with no problems at all. He also didn't pass out.
George Floyd had a heart condition, and it's looking like he died of a heart attack while being held down.
The knee on his neck was incidental to the cause of his death, but because prosecutors were breathing down his neck, the coroner put it in there anyway.
The family hired mercenary was going to say whatever the family paid him to say.
The blood flow and oxygen would have been cut of to his brain by applying pressure to that area of the neck.