A knife found on the person of Richard Neal by the NYPD in 2008. The officer was able to open it with a flick of the wrist.

A Husky utility knife Richard Gonzalez had purchased at Home Depot. He was arrested in April 2011. An officer was able to flick the knife open with one hand.

A knife found on the person of Richard Neal by the NYPD in 2008. The officer was able to open it with a flick of the wrist.

A federal judge ruled on Thursday that the state law that bans “gravity” knives is unconstitutional because it’s too difficult to enforce.

Federal District Judge Paul Crotty specifically mentioned a so-called “wrist-flick test” used by Manhattan District Attorney Cy Vance’s office.

The test, used by cops who attempt to open the knife by flipping it, is arbitrary and shows that the law is too difficult to enforce, the judge said.

“The wrist-flick test is just absurd,” said Martin LaFalce of The Legal Aid Society. “It’s absurd that criminal liability turns on the athletic skill of individual officers.”

Under Vance’s method a police officer who isn’t very good at flicking open a folding knife could give it four or five tries before it locks into place — and it could still be considered a gravity knife, Crotty said.

The ruling pertains to a suit filed by Joseph Cracco, a sous chef who works in Manhattan and who was arrested in 2013 at Grand Central Station for carrying on his belt a folding knife that he uses for work.

Cracco said it took the arresting officer four or five attempts to open the knife with the wrist-flick method. Cracco eventually pleaded guilty to disorderly conduct.

A spokesman for the Manhattan DA said the office is reviewing the decision.

Vance has been outspoken in his opposition to changes in the state gravity knife law, saying that it is needed to keep New Yorkers safe from knife attacks on the streets and subways.