ATM SafetyPIN software is a software application that would allow users of automated teller machines (ATMs) to alert the police of a forced cash withdrawal by entering their personal identification number (PIN) in reverse order. The system was invented and patented by Illinois lawyer Joseph Zingher (U.S. Patent 5,731,575). Contrary to a widely circulated hoax, the system is not implemented in all or even most ATMs.
ATM SafetyPIN software on Wikipedia.
Also see the related article on Snopes.