A 400- pound asthmatic Eric Garner died while being arrested by police in Staten Island. (Photo By: /NY Daily News via Getty Images)

For the second time in as many weeks, a grand jury decided not to indict a police officer for the use of excessive force. This time, however, the weapon of choice was not a gun, but a set of hands, used in an apparent chokehold on an asthmatic Staten Island man named Eric Garner — a tactic that, as it turns out, is forbidden by the NYPD.

This is what the NYPD patrol guide says, per the New York Law Journal:

Members of the New York City Police Department will NOT use chokeholds. A chokehold shall include, but is not limited to, any pressure to the throat or windpipe, which may prevent or hinder breathing or reduce intake of air.

That's explicit. And it has been the department's policy since 1993, shortly after five officers were put on trial (four of whom were acquitted) for killing a 21-year-old by "traumatic asphyxia."

In fact, the NYPD has been warning officers, at least in writing, about the dangers of using chokeholds for decades. In 1985, according to the law journal, then-police commissioner Benjamin Ward limited their use, mandating that they not be used "routinely," and disallowing their use unless an "officer's life is in danger or some other person's life in danger and the choke hold is the least dangerous alternative method of restraint available to the police officer."

But even though the limitation on chokeholds by New York City police officers went from seldom to never, police officers appear to have continuously used the maneuver.

Between June 2013 and July 2014, the Civilian Complaint Review Board, an independent city agency that examines accusations of police misconduct, received more than 200 chokehold complaints. Extend the period back to 2009, and the total number soars into the 1000s. Nearly 200 complaints have been filed on average each year since 2001.

That's not to say that every complaint -- or even the majority -- represented a verified incident in which someone was put in a chokehold by a cop. Indeed, roughly half of claims have been found by the review board to be unsubstantiated. Only a tiny fraction -- around 2 percent -- have been substantiated. A substantial number of cases, meanwhile, are neither fully proven nor disproved.

But even if a minority of cases involve chokeholds, it raises an important question: Why are police officers in New York City still allegedly using the maneuver if it is explicitly banned?

The answer might be because the department is not enforcing the rule stringently. A recent study (pdf) by the review board says that:

Put simply, during the last decade, the NYPD disciplinary decisions in NYPD administrative trials of chokehold allegations failed to enforce the clear mandate of the Patrol Guide chokehold rule. In response to these decisions which failed to hold offending officers accountable, the CCRB and NYPD Department Advocate’s Office [internal affairs] failed to charge officers with chokehold violations pursuant to the mandate of the Patrol Guide chokehold rule.

By failing to properly punish officers who have used a banned method of apprehension, the department effectively shapes the understanding of the rule by officers, the study says.

In essence, in their respective charging decisions, the CCRB and the Department Advocate redefined a “chokehold” to require force to the neck during which an officer actually and substantially interfered with a complainant’s breathing rather than “pressure” to the neck which “may” interfere with breathing. In this respect the chokehold rule “mutated” to adapt to the NYPD disciplinary process, rather than the disciplinary process following the NYPD rule.

Defenders of the officer involved say he was using a proper technique -- one he had learned in the academy that stays away from a person's throat and neck. The New York City medical examiner's office has called Garner's death a homicide resulting from "compression of chest and prone positioning during physical restraint by police.”

Still, it's clear that the police have not closely followed a rule that requires that they "stay away from the neck." It's possible that a lighter and less-asthmatic man might have survived the apparent chokehold -- as some police officers argue -- but that is not the point.