Big Red Switch n.
[IBM] The power switch on a computer, esp. the `Emergency
Pull' switch on an IBM mainframe
or the power switch on an IBM PC where it really is large and
red. "This !@%$% bitty box is
hung again; time to hit the Big Red Switch." Sources at IBM
report that, in tune with the company's passion for TLAs, this is often abbreviated as `BRS' (this has
also become established on FidoNet and in the PC clone world). It is alleged that the emergency
pull switch on an IBM 360/91 actually fired a non-conducting bolt
into the main power feed; the BRSes on more recent mainframes
physically drop a block into place so that they can't be pushed
back in. People get fired for pulling them, especially
inappropriately (see also molly-guard). Compare power cycle, three-finger salute, 120 reset; see also scram switch.