In the cellphone video of George Floyd’s death, the arresting officer, Derek Chauvin, keeps a knee pressed on the back of his neck for about eight minutes until Mr. Floyd stops speaking or moving.
“You don’t have to sit there with your knee on his neck,” exclaimed a bystander off-camera, addressing the officer in language salted with expletives. “He is enjoying that. You are. You are enjoying that. You could have put him in the car by now.”
For police trainers and criminologists, the episode appears to be a textbook case of why many police departments around the country have sought to ban outright or at least limit the use of chokeholds or other neck restraints in recent years: The practices have led too often to high-profile deaths.
“It is a technique that we don’t use as much anymore because of the vulnerability,” said Mylan Masson, a former police officer who ran a training program for the Minneapolis police for 15 years until 2016. “We try to stay away from the neck as much as possible.”
The full details of what happened have yet to emerge, in particular what police body cameras might show about any altercation between Mr. Floyd and Mr. Chauvin, 44, a 19-year veteran of the department who has since been fired. Department records indicate, however, that the Minneapolis police have not entirely abandoned the use of neck restraints, even if the method used by Mr. Chauvin is no longer part of police training.
The manual of the Minneapolis Police Department states that neck restraints and chokeholds are basically reserved for when an officer feels caught in a life-or-death situation. There was no apparent threat of that nature in Mr. Floyd’s detention.
Experts viewing the footage suggest that it was more likely a case of “street justice,” when a police officer seeks to punish a suspect by inflicting pain for something done to the officer during the arrest.
Criminologists viewing the tape said the knee restraint not only put dangerous pressure on the back of the neck, but that Mr. Floyd was kept lying on his stomach for too long. Both positions — the knee on the neck and lying face down — run the risk of cutting off someone’s oxygen supply.
“Keeping Mr. Floyd in the facedown position with his hands cuffed behind his back is probably what killed him,” said Seth W. Stoughton, a former police officer who studies policing and is a professor at the University of South Carolina School of Law. Police training started emphasizing avoiding that prone position about 20 years ago, he said.
In terms of chokeholds, those departments that still allow them usually stress using a kind of wrestling hold, in which the officer wraps his arm around the person’s neck and applies pressure, he said. The idea is to subdue them as quickly as possible in order to get them into a squad car, not to leave them in that possibly deadly position for minute after minute, as happened with Mr. Floyd.
In addition, applying the knee to the back of the neck rather than to the sides risks killing or seriously injuring someone by cutting off the air supply or damaging the cervical spine and other delicate bones in the neck, Mr. Stoughton said. No department permits such a technique in ordinary circumstances, he and others said.
The manual for the Minneapolis police calls a chokehold a “deadly force option” and neck restraints a “non-deadly force option.” Neck restraints involve compressing one or both sides of a person’s neck with an arm or a leg without cutting off the air flow through the trachea. A chokehold is meant to cut off someone’s air supply if the officer feels his or her life is threatened, the manual says.
The manual further explains that the conscious neck restraint may be used against a subject who is “actively resisting,” while rendering the person unconscious should be limited to someone who is aggressive or “for lifesaving purposes.”
John Elder, a spokesman for the Minneapolis Police Department, did not respond to a query about whether the knee restraint used by Mr. Chauvin corresponded to those guidelines.
Many police departments, including the one in Minneapolis, stopped teaching the knee restraint technique and also sought to limit the use of chokeholds after the highly publicized death of Eric Garner in 2014 at the hands of the New York Police Department.
The medical examiner ruled Mr. Garner’s death a homicide caused by the compression of his neck from a “chokehold” and the compression of his chest held on the ground in a prone position. Officer Daniel Pantaleo, who held Mr. Garner in a chokehold, was fired but not charged, inciting protests nationwide.
In Minneapolis, the law enforcement training course that Ms. Masson directed at Hennepin Technical College stopped teaching the knee restraint technique to aspiring police officers after the Garner case, she said, adding that veteran officers should also have learned of the change.
Students in the two-year degree program required of all prospective officers, she said, were instead taught to apply pressure across the upper back. “As soon as the threat is gone, you stop the force, whatever it might be,” she said.
Department records, however, show that such restraint techniques have continued to be used in Minneapolis, although they are sometimes called by different names. In 2012, there were 79 occurrences and in 2013, there were 69. That dropped to 40 in 2018 and was back up to 56 last year. The technique was used against African-Americans far more than other groups, the records show.
Carl Takei, a senior staff lawyer at the American Civil Liberties Union who focuses on police practices, said departments that still allowed chokeholds try to differentiate between cutting off the flow of blood, which renders someone unconscious, and cutting off the flow of oxygen, which is deadly.
“There is still a significant risk that attempting to cut off the flow of blood will also cut off the flow of air,” he said, which was why the A.C.L.U. opposed the technique. “Chokeholds should be banned across the board.”
The fact that Mr. Chauvin kept applying pressure when Mr. Floyd was no longer struggling made it appear to be a case of an officer trying to punish a suspect for doing something that the police did not like — which could include resisting arrest, spitting or insulting an officer, experts said.
If it was a form of “street justice,” that is considered a form of bullying that police academies also instruct against. “It is teaching someone a lesson — next time you will think twice about what you do,” said Philip M. Stinson, a former police officer turned criminal justice professor at Bowling Green State University.
Andy Skoogman, the executive director of the Minnesota Chiefs of Police Association, said the group’s 300 members were appalled by the tactic the officer used in the Floyd case and the “lack of empathy” he showed.
“Bottom line: any type of use-of-force technique must stop when compliance is achieved,” he said, adding that he did not know of any police department in the state that trained in that technique.
Mike Baker contributed reporting. Susan Beachy contributed research.