NEW YORK (AP) – When daniel penny The 25-year-old veteran who wrapped his arm around a homeless man’s neck on a Manhattan subway last year appeared to be using a non-lethal strangulation method long practiced on U.S. Marines.

According to Marine Corps combat instructor Joseph Caballer, who trained Penny in various types of holds, when done correctly, the maneuver should knock out a person without killing them. However, when applied for too long, the technique can restrict blood flow to a person’s brain and end the person’s life within minutes.

“When the person loses consciousness, that’s when you have to let them go,” Caballer told the jury on Thursday.

His testimony comes weeks before the trial of Penny, who faces charges of involuntary manslaughter and criminally negligent homicide after subjecting homeless man and Michael Jackson impersonator Jordan Neely to fatal strangulation last May.

Prosecutors allege Penny “went too far” in her attempt to restrain Neely, showing “indifference” to his life even after he lost consciousness and stopped fighting back.

Penny, an architecture student who served four years in the US Marine Corps, told police she was trying to protect herself and other passengers on the train from a man who was behaving erratically and frightening passengers with disturbing comments. His attorneys highlighted Neely’s previous arrests as well as his struggles with mental illness and drug abuse.

In bystander video of the encounter, Penny can be seen pressing her bicep against Neely’s neck and placing her other arm above her head; The man remained in this position for close to six minutes, even after he limped off.

This technique — an apparent “blood asphyxiation” — can leave the person feeling like they’re “trying to breathe through a crushed straw,” Caballer said. Caballer recalled telling his fellow sailors during his own training sessions: “You don’t want to keep holding on. This could result in actual injury or death.”

When asked by prosecutors if Penny had used chokeholds “inappropriately,” Caballer said she had.

Penny’s attorneys argued that their client attempted to restrain Neely by subduing him, but did not apply strong force throughout the interaction. They cast doubt on the city medical examiner’s finding that Neely died of asphyxiation and pointed to health problems and drug use as possible factors.

Pressed by Penny’s lawyer, Caballer admitted that he “couldn’t tell for sure how much pressure was actually being applied by watching the video.” But he said it sometimes turned out that Penny was using a hold that could cut off blood flow to Neely’s brain.

“He could probably be cutting one of his jugular veins,” the witness added.

In the afternoon, city medical examiner Dr. Cynthia Harris reiterated that she determined he died from lack of oxygen due to asphyxiation. Although he did not fully explain the process of drowning, he stated that “blockage of both arteries in both arteries can kill a person within seconds.”

Jurors were also shown video for the first time Thursday of Penny demonstrating the chokehold to detectives during an interview at the police station.

“He had his back to me, I grabbed him, dropped him to the ground, and he’s still writhing and freaking out,” Penny said. “At one point he had a burst of energy and that’s what I did. We gotta hold him a little bit more still.”

Harris is expected to be the final witness called by prosecutors in a trial that has divided New Yorkers and shed national light on the city’s response to crime and disorganization in its transportation system. It’s unclear whether Penny will take the stand.

In the eighteen months since the murder, Penny has been embraced by conservatives as a good Samaritan who used her military training to protect her mounted friends. U.S. Rep. Matt Gaetz, whom President-elect Donald Trump nominated for attorney general this week, described him as the “Metro Superman.”

But the hearing also saw almost daily protests from Black Lives Matter activists, who labeled Penny a racist vigilante who overreacted to a Black man suffering from mental health issues.

If convicted, Penny faces up to 15 years in prison.