The department I work for would 100% not allow for the pinning of a suspect by the neck, using a knee, a choke hold, or anything else for that matter. That is an old, out of date, and dangerous tactic. Apparently, Minneapolis PD allows for the use of that tactic by specifically trained officers within the department. That said, Chauvin should have removed his knee the instant that Mr. Floyd stopped resisting.
My take is that Chauvin let his ego get the best of him and that in his horribly misguided effort at impressing his authority over his trainee and the other officers on-scene, he kept his knee on Mr. Floyd’s neck/upper back.
As soon as a suspect is in police custody (example: handcuffed), the well-being of the suspect is in the hands of the police. Regardless of what training Chauvin had received/was certified for, as soon as Mr. Floyd stopped resisting he should have ended the restraint. Furthermore, the officers on scene should have immediately moved Mr. Floyd into the recovery position the second he stopped responding.
I’m not a doctor or a medical examiner but it clearly appears that Chauvin was at least a contributing factor in the death of Mr. Floyd, and he should be held responsible for that based upon the investigation and the trial.