To breathe, humans must be able to expand their rib cage and move their diaphragm. Being face down “splints” the diaphragm and makes it hard to breathe. Being in handcuffs makes it worse.
George Floyd was face down with his hands cuffed behind his back, and he had the weight of two grown men on his chest. Like being in the coils of a boa constrictor, he was unable to breathe effectively. Predictably, he suffered cardiac arrest from acute asphyxia.
Because in the past, so many face down and “hog-tied” prisoners have died in custody, police are trained to put them on their side. Derek Chauvin could not have been ignorant of that. And by being directly on Floyd’s chest, Chauvin must have been aware of the changes in Floyd’s breathing, from gasping and broken speech, to weak heaving, to no breathing at all. Even in cardiac arrest, Floyd’s handcuffs weren’t removed for the paramedics to render care.
Much has been made of Chauvin’s knee on the neck. People need to be very clear about what killed George Floyd. He was slowly suffocated. The knee just made it that much more cruel.