When the case started, the defense was going to focus more on what was in the officer’s body than on his mind. Officer Ettienne had been suspended for using steroids  legally, he says, with a doctor’s prescription. The defense lawyer, Adrian Lesher of the Legal Aid Society, argued last year that steroids might have created irrational rage in Officer Ettienne.

Then Mr. Lesher tracked down comments Officer Ettienne had made on the Internet about video clips of arrests. An officer should not have punched a handcuffed man, Officer Ettienne wrote. “If he wanted to tune him up some, he should have delayed cuffing him.”

He added: “If you were going to hit a cuffed suspect, at least get your money’s worth ’cause now he’s going to get disciplined for” a relatively light punch.

“I’m not going to say it was the best of things to do in retrospect,” Officer Ettienne said. “You want to run your mouth with the best of them. As the lawyer Ron Kuby says, stupidity on the Internet is there for everyone to see for all times in perpetuity. That’s the case for me. There were hundreds of comments I made that were positive.”

Officer Ettienne said he has never been disciplined for brutality.

From the defense side, the mouth-running was a gift outright. “It supported our theory of the case  this guy was motivated to cover up his use of excessive force,” Mr. Lesher said.

The prosecutor, Kevin James, tried to persuade the judge, Joel M. Goldberg, that remarks like the one about “Training Day” had nothing to do with the arrest. “It goes into artistic interpretations to a movie, directorship, actors,” Mr. James said.

“I don’t think it’s enlightening.” The judge replied, “If you want to redirect and the witness says I liked it because of the cinematography, he can say that.”

Officer Ettienne said he is now being careful to mask his identity on the Web and that he has curbed his tongue because of the acquittal. “I feel it’s partially my fault,” he said. “It paints a picture of a person who could be overly aggressive. You put that together, it’s reasonable doubt in anybody’s mind.”