The term dead-man's switch dates back to the infancy of the railroads. Trains do not use a gas pedal like automobiles, but a throttle control you set - similar to cruise control. Because early railway engineers often worked long hours over desolate country, many accidents were caused when they fell asleep at the controls - resulting in a runaway train

To prevent this a switch was developed that the engineer had to sit or step on - else the train would stop. Of course it's also true that if the engineer had a heart attack and died, then the train would stop. And this "dead man" scenario lent us a much more romantic name - and kept the pride and reputation of railroad engineers intact.

The original dead man's switch was a simple mechanical switch - usually a pressure plate. In modern trains, every 30 seconds an alarm sounds and a light flashes red. The engineer must put his hand over an optical sensor to reset the alarm. If the alarm is not reset, the locomotive will automatically begin braking.

Today this type of safety switch has many names, though the principle remains the same. It may be called a "positive on-off control" or a "constant pressure switch" or an "enabling device". In official railroad parlance, the dead man's switch is now a Safety Control Foot Pedal.

These switches can be found on virtually any device that has an electric or gas combustion engine - or any device that can cause safety problems if the operator is inattentive; from power hand tools to lawn mowers to high voltage insulation testers and even baby strollers. Would-be hijackers or robbers often claim to have a bomb rigged to go off if they are killed - just another application of the dead man's switch.

Recently the term has also been applied to a type of software application - one that will "clean up" after the user in case of death. ArsWare has one such program, creatively titled, Dead Man's Switch that, if not reset by a given time, will automatically carry out a series of tasks - such as posting messages to websites, sending e-mails to loved ones (or hated ones), and encrypting or destroying sensitive files.