Deadman's switch
A dead man's switch (or dead man's handle) is a device intended to take some action if the human operator becomes incapacitated in some way, a form of fail-safe practice.
For example, most freight elevators and lawn mowers use a dead man's switch or a similar mechanism, causing them to respectively stop closing the elevator door or shut down if the switch is released. If a person faints while mowing the lawn, their hands will relax and the mower will shut off.
A very common use is in a locomotive, especially those used on underground railway systems or other rapid transit systems. The driver is required to hold down the dead man's handle to keep the train running - if he is unable to do so, the train will brake and come to a stop. The recent inquiry into the Waterfall train disaster in Sydney, Australia found that it was common practice to wedge the dead man's handle (in fact a footplate) down using an umbrella or signalling flag, since the drivers found it tiring to apply pressure continually. Naturally this compromised safety, and has been one of the factors found to have caused this crash.
Hence, a more elaborate design of a dead man's switch requires the operator not only to keep the switch pressed but also to release it periodically, say, at least once every 30 seconds. Such a system is in mandatory use in the locomotives in some countries' railway networks (for example the "SiFa" (Sicherheitsfahrschaltung) of German railway locomotives). In many modern New York Subway trains, for example, the dead man's switch is incorporated into the train's speed control (on the R142A, the lever must be continually held in place by the train operator; because the lever is also used to both accelerate and stop the train, it must remain in constant use).
Dead man's switch devices have also been used in suicide bombing, to trigger the explosive if the bomber is shot or overpowered. This is a fail-deadly mechanism, rather than a fail-safe mechanism.
