They call it "contagious shooting". One cop fires. Then the others, believing the shooting is itself evidence of a threat, follow suit. The bullets start flying in rapid succession.
The consequences, when the target turns out to be an unarmed black man in New York, come soon after. They could call that "contagious rage".
Communities, marginalised by poverty and discrimination, (New York is full of rich, white people but somehow the police never shoot them by accident; they rarely even arrest them on purpose) demand answers and change. The former does not happen without a fight and even then only partially; the latter rarely comes at all.
Seven years ago the city was almost ripped apart when the New York Police officers unleashed 41 bullets at Amadou Diallo, who was unarmed as he tried to get in to his apartment building. This weekend they fired 50 shots at a car of unarmed men, killing Sean Bell, 23, as he left a stag night party at a strip club with two friends on the morning of his wedding day.
The shooting occurred when undercover policemen opened fire on Mr Bell as he drove away from the club. It is not yet clear why and the five policemen in question have been put on administrative duty. Protesters have said the only difference between the two incidents is 9 bullets. But something else seems to have changed in the intervening seven years. The racial politics of New York.
While the shootings bare comparison they are different in crucial ways. First, Mr Diallo was shot by four white officers; of the five who opened fire on Mr Bell two were black, two were white and one was Hispanic. Mr Bell is just as dead as Mr Diallo and for no better reason. But the racial composition of the policemen in question shifts the discussion from racism in general to policing in particular.
But the city has changed. It hasn't eliminated racism. A quick trip around East New York or the North Bronx will settle any illusion of that. But it manages racially-charged incidents far better than it did in 1999. Both Giuliani and the NYPD, who had brutally sodomised Abner Louima, a Haitian immigrant two years earlier, already had a terrible relationship with African Americans. Nonetheless, Giuliani dismissed accusations of racism in the Diallo case out of hand, refused to even meet with black elected officials for a month and refused to entertain any public criticism of the police actions.
Bloomberg by contrast called Mr Bell's fiancee the evening of the shooting, and has been in constant contact with black elected officials and leading figures in the black community. Yesterday he branded the shooting "excessive" and "unacceptable".
Whether this will be enough to quell the anger of the shooting and whether it will have any consequences in terms of justice for Mr Bell - his two companions were seriously injured - is another matter.
The calls for calm at public rallies in Queens were heckled by local protesters. When one councilman declared, "We are not going to be angry," the crowd responded: "Oh yes we are!"