Trying to block an airway and four carotid artery branches with just one knee is plenty tough.
An acute occlusion of one common carotid artery can cause a stroke...and it can be fatal.
Also, when professional fighters apply a triangle blood choke, they only apply direct pressure to one side of the neck. The other side is constricted by the person's own shoulder, arm or collarbone area. In this case, it may be possible that the guy's own shoulder or the street for that matter could have applied enough pressure on the underside of the neck to cause him to go unconscious. We don't know yet until an autopsy is performed though.
"Hey jackass, get your government off my freedom." MOLON LABE
