When a thief forced you to take
Eg: If your PIN is 1234, you punch 4321.
The moment you the reverse, will come out, but will be stuck into the machine half way out and it will alert the police without the notice of the thief.
Every ATM has it; It is specially made to signify danger and help. Not everyone is aware of this from the ATM, do not argue or resist, you might not know what he or she might do to you. What you should do is to punch your PIN in the reverse...