The original email goes something like this:
PIN NUMBER REVERSAL (GOOD TO KNOW)
If you should ever be forced by a robber to withdraw money from an ATM machine, you can notify the police by entering your Pin # in reverse.
For example if your pin number is 1234 then you would put in 4321. The ATM recognizes that your pin number is backwards from the ATM card you placed in the machine.
The machine will still give you the money you requested, but unknown to the robber, the police will be immediately dispatched to help you.
This information was recently broadcast on TV and it states that it is seldom used because people don't know it exists
There is also another variation:
See this important message below.
I just found out that should you ever be forced to withdraw monies from an ATM machine, you can notify the police by entering your Pin # in reverse. The machine will still give you the monies you requested, but unkown to the robber, etc, the police will be immediately dispatched to help you. The broadcast stated that this method of calling the police is very seldom used because people don't know it exist, and it might mean the difference between life and death. Hopefully, none of you will have to use this, but I wanted to pass it along just in case you hadn't heard of it. Please pass it along to everyone possible.
Unfortunately, even though it seems like a good idea, both of the emails are untrue. The technology that makes this possible is not implemented (at least not on most ATM machines). It was proposed back in 1994 by Joseph Zingher from Chicago who created an ATM software that would silently call police if a PIN was entered in reverse. The idea is also patented - U.S. Patent 5,731,575. Unfortunately, the banks have no legal obligation to do so and showed no interest in the software. In 2006 the email of above contents started circulating around the Internet. No need to forward it, the information is untrue.
The idea is interesting though - but if you want to protect your money - here is another idea (I do it like this personally):
Open an Internet banking option with your bank. Use 2 accounts and transfer only a small amount of money (you can do that once or twice per month) using the Internet banking option to your every-day account. If you happen to come across the robber situation, you will be only allowed to withdraw a small amount of money. The idea to keep only small amount of money available may be probably implemented with your credit card as well.