According to Wikipedia, a death switch is “an automated program by which a computer regularly probes a subscriber. The subscriber is required to make a response — consisting of logging with a secret password — to prove that she is still alive. When the subscriber fails to make a response for a certain amount of time, the program assumes she is dead and emails out pre-scripted messages to her pre-defined recipients.”
In the last issue of Nature, David Eagleman wrote a paper about death switches:
"It soon became appreciated that death switches provided a good opportunity to say goodbye electronically. Instead of sending out passwords, people began programming their computers to send e-mails to their friends announcing their own death. “It appears I’m dead now,” the e-mails began. “I’ll take this as an opportunity to tell you things I’ve always wanted to express…”
Soon enough, people realized they could program messages to be delivered on dates in the future: “Happy 87th birthday. It’s been 22 years since my death. I hope your life is proceeding the way you want it to.”
With time, people began to push death switches further. Instead of confessing their death in the e-mails, they pretended they were not dead. Using auto-responder algorithms that cleverly analysed incoming messages, a death switch could generate apologetic excuses to turn down invitations, to send congratulations on a life event, and to claim to be looking forward to a chance to see them again sometime soon."