Source : Free On-Line Dictionary of Computing

Big Red Switch
     
        [IBM] 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."  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
        {IBM PC} 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}.