The fail-safe was a mechanism in the Swan which, when triggered by the turn of a key, would terminate the electromagnetic buildup which the button was meant to discharge. The switch was located in a large crawlspace beneath the station accessible by one of a number of grates in the floor. Kelvin Inman, drunk and looking for an escape from pushing the button, revealed the fail-safe to Desmond, saying that it was the only other way out, and that turning the key would make it "all go away". ("Live Together, Die Alone")
On the day of the system failure coinciding with the crash of Flight 815, Desmond acquired the fail-safe key by ripping it off of Kelvin's neck, after having accidentally killed him. The key was imprinted with the Swan's logo, and Desmond kept it inside his copy of Our Mutual Friend. It is unknown how many DHARMA Initiative personnel were aware of the fail-safe, although it is highly likely at least Radzinsky, as the station architect, was aware of its existence. ("Live Together, Die Alone") The Fail-safe device appeared to have some strange glowing light eminating from around the keyhole. It was possibly the same light from the electromagnetic pocket, similar to the heart of the island.
After Locke caused another system failure by destroying the computer, Desmond retrieved the key and triggered the fail-safe mechanism, finally terminating the electromagnetic anomaly. This action resulted in the events of the discharge, the complete destruction of the station, and Desmond becoming unstuck in time. ("Live Together, Die Alone") ("Further Instructions") ("Flashes Before Your Eyes")
Desmond himself as fail-safe
According to Charles Widmore, Desmond was a fail-safe himself, because of his unique resistance to electromagnetism. He was Jacob's last resort in case the Man in Black managed to kill all of the candidates -- one final way to make sure that his brother would never leave the Island. ("What They Died For")
- The key used for the prop was an Ilco ORION key.
- For fan theories about these unanswered questions, see: Fail-safe/Theories
- What would have happened if Desmond actually never turned the fail-safe?