The Jargon File (version 4.4.7, 29 Dec 2003):

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; see also scram switch.


The Free On-line Dictionary of Computing (30 December 2018):

Big Red Switch
BRS

    (BRS) IBM jargon for the power switch on a computer,
   especially 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."

   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.

   [Jargon File]

   (2014-08-10)