That is the least important part, seeing as it had nothing to do with his actual death. Floyd died of an overdose and the cop's standard restraint that is employed every day without any deaths is irrelevent. The technique does not restrict breathing in any way due to the pressure being exerted on the back of the neck. The trachea is protected by several underlying structures including the vertebrae. If Chauvin had exerted enough force to restrict his breathing the damage to the underlying structures would have been evident in the autopsy.