A video that went viral of panic on a crowded rush-hour subway train last week is offering a lesson for riders in what not to do. Transit Reporter Jose Martinez explains.

They're inside every subway car. A chord that can bring a roaring subway train to a screeching stop.

"It seems awfully easy to pull," said one straphanger. "It seems like you could be going to catch yourself if the train was stopping suddenly, and you could actually pull on it."

Panicky riders on a Brooklyn-bound D train learned last week that pulling the emergency cord usually is a bad idea.

A rider activated the brake when worms and crickets went flying across a car during a subway stunt gone wrong as the train crossed the Manhattan Bridge.

Lawyer Matthew Moffa was among those stuck on the train when it was brought to an abrupt stop.

"I knew pretty quickly someone had pulled the emergency brake," Moffa said. "We were talking about it in my seat. But I didn't know why."

The rider who pulled the brake had good intentions but was wrong, officials said.

That's because an emergency stop usually will "delay" the arrival of help.

MTA spokesman Kevin Ortiz says the brake should be activated only as a last resort.

"Use the brakes if someone is caught in a closing door or is being dragged by a moving train," Ortiz said.  "Do not use the emergency brake between stations," he says.

Moffa agrees.

"Every time somebody pulls the emergency brake, it's usually a bad idea," he said. "I mean, people have all sorts of false impressions about what that brake should do. And I thought, well, if they had just not pulled the brake, we all could have gotten off that train a lot quicker."

As for Zaida Pugh, who brought those crickets and worms onto the train, causing panic, she turned herself into police on Tuesday.

The MTA says it doesn't have an exact figure for how many times riders have pulled the emergency brake on subway cars, saying only it doesn't happen very often. Small consolation for riders who were stuck on that D train with Pugh, her worms and her crickets.

"I got home half an hour late and we were up there for at least 20 minutes," Moffa said.

Pugh has said she was trying to draw attention to how the homeless are treated in the subway.