A former Marine who trained Daniel Penny to apply a chokehold said Thursday that images and video suggest that he might have done so improperly when he killed a homeless man last year.

The trainer, Joseph Caballer, testified that photos seemed to show Mr. Penny trying to use a “blood choke” to restrain the man, Jordan Neely, on the floor of a subway car. A proper blood choke cuts off oxygen to the brain in as little as eight seconds, said Mr. Caballer, who taught Mr. Penny the technique when they served together in the Marine Corps.

But as the men struggled and Mr. Neely shifted in Mr. Penny’s arms, Mr. Caballer said, the hold could have turned into an “air choke,” which takes longer to render a person unconscious and could “cause injury to the trachea or windpipe.” The hold is not taught by the Marines, he told jurors in Manhattan criminal court.

Mr. Neely, who had a history of mental illness, had boarded an uptown F train on the afternoon of May 1, 2023, and begun yelling, throwing his jacket on the floor and striding through the subway car, according to witnesses.

Mr. Penny said he had stepped in to protect other riders as the train traveled between stations. He did not put pressure on Mr. Neely’s neck, he told the police, and restrained him only to keep him “from hurting anybody else.”

Mr. Penny has been charged with manslaughter and criminally negligent homicide.

Prosecutors with the Manhattan district attorney’s office said his actions became criminal when he did not release his chokehold once Mr. Neely was no longer a threat, when the train had stopped at the Broadway-Lafayette Street station in Manhattan and the doors had opened, letting people out of the subway car.