On old IBM mainframes, a big red button was available to the operator which was directly connected to the power supply. Pressing this button was equivalent to the modern interrupt 110, in that it would power down the system, no ifs, ands, or buts. In some very old systems, the button may not have simply cut the power, but may have driven an explosive bolt through the power supply.
Tales say that this button was to prevent the development of AI in these machines, hardly a likely happenstance, but good marketing.
Further rumours suggest that some secure modern systems have similar buttons, which will scram the main memory and halt processing if necessary. (E.g., men in black balaclavas carrying Kalashnikovs storm the machine room.)
This is an alternate spelling of big red switch, from the Jargon File.