Key takeaways from the Derek Chauvin trial, Day 7

A nationally renowned police use-of-force expert took the witness stand on Tuesday in the murder trial of Derek Chauvin, testifying that the former officer, in his opinion, used excessive force in the death of George Floyd.

Sgt. Jody Stiger, a former Marine and a 28-year veteran of the Los Angeles Police Department, was called as the prosecution's first expert witness in the high-profile case.

Stiger said he reviewed all of the videos showing the May 25, 2020, fatal encounter with Floyd -- including footage from police body cameras, surveillance cameras and videos taken by bystanders -- and analyzed investigative reports and the Minneapolis Police Department's use-of-force policies to draw an opinion about the actions taken by Chauvin and other officers involved in the arrest.

"My opinion was that force was excessive," Stiger testified.

As an expert witness, Stiger said he is being paid a $10,000 flat rate, plus an additional $2,950 for his courtroom testimony in the case.

He said the offense Floyd was being taken into custody for, allegedly using a fake $20 bill to purchase a pack of cigarettes, was a low-level offense.

"Typically, you wouldn't expect to use any kind of force," he said.

Stiger testified that initially the officers were justified in using force when Floyd, who was in handcuffs, balked and resisted getting in the back of a patrol car.

"However, once he was placed in the prone position on the ground, he slowly ceased his resistance, and at that point the officers -- ex-officers, I should say -- they should have slowed down or stopped their force as well," said Stiger, a former member of the LAPD's Use of Force Review Board.

Based on his review of Floyd's arrest, "I believe they (the officers) felt that he was starting to comply and his aggression was starting to cease," Stiger testified.

Stiger is expected to return to the witness stand on Wednesday to continue his testimony and face cross-examination from Chauvin's attorney, Eric Nelson.

Police trainers testify

A series of Minneapolis Police Department trainers also took the witness stand on Tuesday, testifying that Chauvin had completed multiple refresher courses on everything from crisis intervention and de-escalation to cardiopulmonary resuscitation.

But the police trainers, testifying for the prosecution, said the techniques Chauvin was seen using to control Floyd, including grinding his knee into the man's neck for 9 minutes and 29 seconds, is not something they train Minneapolis police officers to do.

Sgt. Ker Yang, a 24-year veteran of the department and its crisis intervention training coordinator, testified that he recognized Chauvin through "training." He confirmed records that said in 2016 Chauvin participated in a 40-hour course on crisis intervention, where professional actors presented officers with scenarios of people suffering a variety of emergencies, including those prompted by mental illness and drugs.

Yang outlined the department’s critical decision-making model, which includes applications for use of force and crisis training.

He explained that a “crisis” could be “any event situation that is beyond a person's coping mechanism, and ... what is beyond their control."

"We train them to assist a person to bring them back down to their pre-crisis level,” said Yang, who holds a master's degree in counseling psychology and a doctorate in general psychology.

He said it is crucial for officers to be constantly reassessing the circumstances of a use-of-force encounter as it is unfolding.

Yang said if the person in crisis is in need of medical attention, it “would be the immediate goal” for officers to “give them medical attention.”

"A lot of the time we have the time to slow things down and re-eval and reassess and go through this model," Yang said.

During cross-examination, Nelson asked if officers are also trained to assess possible threats coming from a hostile crowd of people witnessing the encounter. On numerous occasions during the trial, Nelson has pointed out that bystanders were recording the arrest of Floyd and yelling profanities at the officers to get off him as he repeatedly cried out "I can't breathe."

Yang agreed that part of the department's "Critical Decision Making" model is to evaluate threats going on around them at the time they are trying to take an unruly person into custody. He said that as the intensity of a crisis grows, "the risk to the officer is greater."

Minneapolis Police Lt. Johnny Mercil, a use-of-force training instructor, testified that he helped develop the department's annual in-service use-of-force training program that Chauvin took in 2018.

Mercil explained that officers are trained to “use the lowest level of force possible" to bring a person who is resisting arrest under control in a way that is safe for a subject as well as the officers.

He said officers are also trained to know that some parts of the body are more prone to injuries, such as the neck and the head.

When shown a photo of Chauvin with his knee on Floyd's neck, Mercil said the technique is not currently, and has never been, authorized as a restraint technique taught by the Minneapolis Police Department.

"We tell officers to stay away from the neck when possible," Mercil said.

While being questioned by Nelson, Mercil was shown a freeze-frame of a video that appeared to show Chauvin's knee on Floyd's shoulder blades at the base of his neck, which Mercil said was an approved technique officers are trained on.

Minneapolis Police Officer Nicole MacKenzie, the department's medical support coordinator, testified that Chauvin completed in-service training in CPR and first aid multiple times, including a course in 2019.

MacKenzie testified that officers are trained to begin CPR immediately if they do not detect a pulse, like in the case of Floyd.

Prosecutor Steve Schleicher asked MacKenzie about the phrase "if you can talk, you can breathe," which video evidence shows is what officers said to Floyd as he complained about not being able to breathe.

"That would be incomplete to say," MacKenzie said. "Someone can be in respiratory distress and still able to vocalize it. Just because they can talk doesn't mean they can breathe adequately."

Testimony on 'excited delirium'

During cross-examination, Nelson asked MacKenzie about the training officers receive on the topic of excited delirium.

One of the former officers involved in Floyd's arrest, Thomas Lane, was heard on body camera video played in court expressing concern that Floyd could be suffering from excited delirium.

MacKenzie explained that officers are taught to spot signs of excited delirium and to recognize "that this is a medical condition, not necessarily a criminal matter."

Nelson asked if teaching regarding the subject of excited delirium would also include controlled substances as a possible contributing factor.

"What we're usually teaching is that (in) most of the people experiencing something like excited delirium, usually there's illicit drugs ... that might be a contributing factor," MacKenzie said.

She said a person suffering from psychosis could also experience hypothermia, an elevated heart rate, and "they might be insensitive to pain."

Prosecutors objected to the line of questioning, prompting the judge to excuse MacKenzie for the day. Nelson said that once the prosecution rests its case, he plans to call her back to the witness stand to testify on the excited delirium training officers undergo.

ABC News' Whitney Lloyd contributed to this report.