The boyfriend, an M.B.A. student named Anthony DeMaio, one of about 15,000 people typically charged in New York City each year with attacking a partner, was charged with misdemeanor assault. He was also charged with a new crime: a felony called second-degree strangulation, created in 2010 to help prosecutors fight domestic violence by upgrading a form of assault they say is favored by abusers.

New York’s law, like dozens of choking statutes across the nation, is popular with law enforcement officials. In 2011 in New York City, 1,458 domestic violence assaults that would have been considered misdemeanors under the old law – more than 9 percent of them – were charged as felony strangulation.

But second-degree strangulation – choking to the point of injury, impairment, stupor or unconsciousness – can leave ambiguous marks or no marks at all, making it tricky to prove. “If you don’t know how to follow the bread crumbs it’s very easy to miss,” said Gael Strack, chief executive of the National Strangulation Training Institute, an anti-domestic-violence group based in San Diego. Of the thousands of defendants charged in New York City, fewer than 20 have gone to trial, state officials said. Experts say that thousands of police and medical professionals around the country have not been trained on how to execute the new statutes.

And in Mr. DeMaio’s case, strangulation was a tough sell. I know because I was on the jury.

Choking, experts say, is one of the most pervasive forms of domestic violence, with its overtones of power and control, and one of the best predictors of more serious violence. “A woman who has been choked is seven times more likely to be the victim of a domestic violence homicide later,” Ms. Strack said. Until the new law, though, unless it resulted in serious physical injury, like brain damage, choking could be classified no higher than misdemeanor assault, which carries a maximum sentence of one year but rarely draws more than a few months. In recent years, choking assaults have drawn the attention of lawmakers across the country. New York is one of 30 states that have criminalized that kind of assault.

Under New York’s law, second-degree strangulation is a Class D felony, calling for at least two years in prison – the same as second-degree assault, which requires infliction of serious physical injury.

Wanda Lucibello, chief of the special victims unit in the Brooklyn district attorney’s office, said the new statute “helps us as prosecutors to address what happens when a victim comes in and says, ‘He choked me.’ ” She added, “We now have a whole repertoire of follow-up questions to elicit exactly the results of that action, things that are not as necessarily visible to the naked eye as a punch would be. Were your airways obstructed? Were there changes in your breathing or speech?”

In Mr. DeMaio’s case, though, it did not appear that all evidence had been collected. The victim, Ainsley Gill, was the trial’s first witness. Ms. Gill, a college graduate who works in interior design, said on the stand that she and Mr. DeMaio had been together for a year. He had beaten her on other occasions, she said, but she loved him because she “saw good in him.” On that spring morning in 2011, Ms. Gill testified, he burst into the bathroom and accused her of receiving a text message from another man. When he slammed her head against the tub, she said, she saw stars. And as he squeezed her neck, she said, she thought she was going to die.

Police photos of her injuries taken soon after the assault were projected on a courtroom screen: scalp and hip abrasions, a bruised eye, contusions on her arm, breast and back. A close-up of Ms. Gill’s neck showed a rash of red dots. The officer who responded to her 911 call testified that he had observed bruising and requested an ambulance. But he did not write down that he had seen any injury to her neck, nor did the hospital emergency-room team in its brief written report.

The first professional to flag a possible choking was a police officer from the Domestic Violence Unit, Samantha Sonnett. At the hospital, she said, she saw marks on Ms. Gill’s throat consistent with choking. She testified that when she called Mr. DeMaio, told him that his girlfriend was in the hospital and instructed him to come to the station house, one of his first questions was, “Is she still breathing?”

A forensic pathologist called by the prosecutor testified that the dots on Ms. Gill’s neck were petechiae – broken blood vessels suggesting choking. The defense’s expert, an emergency room doctor, dismissed the dots, saying that if Ms. Gill had been choked, her eyes would have been the first area to exhibit petechiae.

Ms. Strack said in an interview after the trial that in fact, collecting visual evidence of choking was not a simple thing.

“Petechiae can be under the eyelid,” she said. “This type of crime needs very specific training on how you photograph it. Bruising and swelling can turn up 24 or 36 hours later.”

Ms. Gill had testified that her bruising and swelling worsened over time. But no photos were offered to verify this. Ms. Gill’s roommate said she witnessed Ms. Gill tell both the first officer and medical professionals at the hospital that Mr. DeMaio had choked her.

In a telephone interview, Mr. DeMaio’s lawyer, H. Benjamin Perez, said that his client had no comment. But Mr. Perez said the strangulation charge was being overused. “It seems like it’s being tested to see where it can stick,” he said.

Mr. DeMaio did not take the stand. He kept his head down, filling pages of a legal pad.

We the jury were eight women and four men, a mix of white and Hispanic, gay and straight, athletic to sedentary, all educated past high school.

We quickly agreed that Mr. DeMaio had committed misdemeanor assault, defined as injuring someone either intentionally, recklessly or negligently. On the strangulation charge, though, it was as if we’d tuned into different instruments at a concert. I found Ms. Gill, the domestic violence officer, the roommate and the prosecution’s pathologist credible. Other jurors argued that the first officer on the scene and the emergency-room team would never have failed to overlook something as major as choking.

We reviewed the judge’s instructions. Was the victim’s normal breathing impeded? Yes. Was it intentional? I thought so – Mr. DeMaio had asked if she was still breathing. And the red dots on Ms. Gill’s neck, I argued, constituted the necessary “physical injury.”

“That could have happened if he punched her with his watch on and it scraped across her neck,” one juror countered. “Maybe if he dragged her by her hair across the carpet.”

In the end, I went along to avoid causing a mistrial. We found Mr. DeMaio guilty of misdemeanor assault, not guilty on strangulation. The judge said we were the first jury he knew of in Manhattan to try second-degree strangulation. As we filed back into the jury room, some jurors marveled at being legal guinea pigs.

“Can I change my vote?” one half-joked.

Another juror, a British woman as diffident as Miss Marple, met me at the elevator. “We all know he did it,” she said. “We just couldn’t prove it.”

Our verdict, it turned out, fit the pattern: none of the 19 people who have been tried on second-degree strangulation charges in New York City have been convicted of it, according to the state.

At the sentencing in November, Mr. Perez argued that Mr. DeMaio had been “overcharged” and that if he had been charged with only a misdemeanor, he would have probably been let off with probation. Mr. DeMaio read a prepared statement in which he apologized personally to Ms. Gill, though without looking at her.

The judge, Bruce Allen, imposed the sentence prosecutors requested: 60 days in jail – typical for misdemeanors – and 3 years’ probation, anger-management counseling and a 5-year protection order.

Mr. DeMaio was released from Rikers last month after 38 days. His sentence was reduced for good behavior.

Randy Leonard contributed reporting.