Seth Stoughton, an associate professor at the University of South Carolina School of Law, judged Chauvin's actions against what a reasonable officer in the same situation would have done, and repeatedly found that Chauvin did not meet that test.
"No reasonable officer would have believed that that was an appropriate, acceptable or reasonable use of force." Stoughton said of the way Floyd was held facedown with a knee across his neck for up to 9 minutes, 29 seconds.
He said, too, that the failure to take Floyd out of the prone position and render aid "as his increasing medical distress became obvious" was unreasonable.
He said it was unreasonable as well to think that Floyd might harm officers or escape after he had been handcuffed to the ground. And in yet another blow to the officer's defense, Stoughton said a reasonable officer would not have viewed the bystanders as a threat while they were restraining Floyd.
Earlier in the day, Philonise Floyd, 39, took the witness stand and lovingly recalled how his older brother used to make the best banana mayonnaise sandwiches, how George drilled him in catching a football, and the way George used to mark his height on the wall as a boy because he wanted to grow taller.
He shed tears as he was shown a picture of his late mother and a young George, saying, "I miss both of them."
His testimony at Chauvin's murder was part of an effort by prosecutors to humanize George Floyd in front of the jury and make the 46-year-old Black man more than a crime statistic. Minnesota is a rarity in allowing "spark of life" testimony during the trial stage.
Philonise Floyd described growing up in a poor area of Houston with George and their other siblings.
VIDEO: Police chief: Kneeling on George Floyd's neck violated policy
"He used to make the best banana mayonnaise sandwiches. And he used to make the best syrup sandwiches because George couldn't cook, he couldn't boil water," he said.
He said Floyd also played football and deliberately threw the ball at different angles so Philonise would have to practice diving for it. "I always thought my brother couldn't throw. But he never intended to throw the ball to me," he said, smiling.
Earlier Monday, the judge refused a defense request to immediately sequester the jury, the morning after the killing of a Black man during a traffic stop triggered unrest in a suburb just outside Minneapolis.
Chauvin attorney Eric Nelson argued that the jurors could be influenced by the prospect of what might happen as a result of their verdict.
"Ultimately, your honor, the question becomes: Will the jury be competent to make a decision regardless of the potential outcome of their decision?" he said.
VIDEO: ER doctor says he theorized George Floyd most likely suffocated
But Judge Peter Cahill said he will not sequester the jury until next Monday, when he anticipates closing arguments will begin. He also denied a defense request to question jurors about what they might have seen about Sunday's police shooting of 20-year-old Daunte Wright in Brooklyn Center.
In the wake of the shooting, hundreds of protesters broke into about 20 businesses, jumped on police cars and hurled rocks and other objects at officers in Brooklyn Center, about 10 miles from the heavily fortified Minneapolis courthouse.
The Brooklyn Center police chief later called the shooting accidental, saying the officer who fired apparently meant to draw a Taser, not a handgun.
Prosecutor Steve Schleicher argued against sequestering the Chauvin jury, saying it wasn't an effective remedy, and he opposed questioning the jurors: "We can't have every single world event that might affect somebody's attitude or emotional state or anything be the grounds to come back and re-voir dire all the jurors."
The judge previously told the jury to avoid news during the trial.
The ruling came as the trial entered its third week, with the prosecution close to wrapping up its case and giving way to the start of the defense.
Stoughton, the use-of-force expert, said the officers who subdued Floyd should have known he was not trying to attack them when he struggled and frantically said he was claustrophobic as they tried to put him in a squad car.
"I don't see him presenting a threat of anything," Stoughton said, adding that no reasonable officer would conclude otherwise.
Stoughton also pointed to instances when Chauvin should have been aware of Floyd's growing distress: After one officer suggested rolling Floyd onto his side, Chauvin said no. Chauvin ignored bystanders who were shouting that Floyd was not responsive. And when another officer said Floyd didn't have a pulse, Stoughton said Chauvin's response was "huh."
Stoughton's testimony is similar to that offered by previous witnesses but it is framed from the perspective of what a reasonable officer should have done. Officers are allowed to use deadly force when someone puts the officer or other people in danger, and legal experts say a key question for the jury will be whether Chauvin's actions were reasonable.
His testimony also gave prosecutors a chance to again play several snippets of body-camera video of Floyd's pleas for help.
Earlier, Dr. Jonathan Rich, a cardiology expert from Northwestern Memorial Hospital in Chicago, echoed previous witnesses in saying Floyd died of low oxygen levels from the way he was held down by police.
He rejected defense theories that Floyd died of a drug overdose or a heart condition. Floyd had fentanyl and methamphetamine in his system, high blood pressure and narrowing of the heart arteries, according to previous testimony.
"It was the truly the prone restraint and positional restraints that led to his asphyxiation," Rich said.
In fact, the expert said, "Every indicator is that Mr. Floyd had actually an exceptionally strong heart."
Rich said he had reviewed Floyd's autopsy report. He said that some narrowing of the arteries is extremely common, and that Floyd had a mildly thickened or mildly enlarged heart that would be normal in someone with high blood pressure.
Rich said that when one officer said that Floyd was passing out, police probably could have saved his life by repositioning him so his lungs could expand again. And once an officer noted that Floyd had no pulse, police still had a significant opportunity to save his life by administering CPR, he said.
On cross-examination, Nelson tried to shift blame onto Floyd, asking Rich if Floyd would have survived if he had "simply gotten in the back seat of the squad car."
But Rich quickly reiterated the death was caused by the officers' actions: "Had he not been restrained in the way in which he was, I think he would have survived that day. I think he would have gone home, or wherever he was going to go."
Derek Chauvin, 45, who is white, is charged with murder and manslaughter in Floyd's May 25 death. Police had been called to a neighborhood market where Floyd was accused of trying to pass a counterfeit bill.
Prosecutors say Chauvin had his knee on Floyd's neck as he lay pinned to the pavement for 9 1/2 minutes. Bystander video of Floyd crying "I can't breathe!" until he finally went limp sparked protests and scattered violence in Minneapolis and around the U.S.
Chauvin's attorney is expected to call his own medical experts to make the case that it was not the officer's knee that killed Floyd. The defense has not said whether Chauvin will testify.