The emergency cord.
Don't pull it if there is a fire. Don't pull it if there is a medical emergency. Don't pull it if there's a crime. In fact, according to the sign, "Do not pull the emergency brake." Period.
Straphangers have long been amused and befuddled by the seemingly backwards instructions on when and when not to use the emergency brake on the subway. Pulling the cord brings the train to an immediate stop, locking you in the car somewhere underground until MTA notify police, who then can come to the rescue. This process can take more than a little time, thereby having the exact opposite effect of what you want (like, say, getting away from a murderer with a knife).
That's where Casey Neistat comes in.
The artist and filmmaker decided to make his own PSA explaining just when, exactly, you should yank the cord.
After riding the rails, quizzing passengers and cops, and getting himself arrested, Neistat uncovers the "golden rule."
WATCH to find out what that rule is: