/B-R-S/
[ IBM ]
n. 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 acronymized 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 machines 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.