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The Big Red Switch (BRS) is a physical or metaphorical switch or button for which activation has ominous implications. A Big Red Switch is always very visible, and may stand as a warning itself as it is often designed to only be used in extreme situations. As such, the term has become a metaphor for extreme situations such that when an emgerency arises, someone might say "pull the Big Red Switch."
Most often the Big Red Switch is a last resort in computer security, most specifically in mainframes or servers that have come under an attack that cannot be stopped and thus must be shut down. The term may also refer to major system resets.
The Big Red Switch may also be called the Big Red Button (BRB).
The emergency pull switch on old IBM mainframes were big and red, as was the power button on old IBM PCs. Oftentimes these buttons were used to prevent further catastrophe. However, the origins of the term may go even farther back, as the image of a red switch or button was widely used during the Cold War era, when nuclear missile launch buttons were often portrayed in this manner.
Literal Big Red Switches now have a "molly-guard", which prevents accidental activation of the switch. It came to be known as such when the young daughter (Molly) of a programmer pulled the Big Red Switch, shutting down an IBM 4341 mainframe twice in a single day.