Big Red Switch

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 {TLA}s, 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}.