Source : Free On-Line Dictionary of Computing

Programmer's Switch
     
         A button on the front of some {Apple Macintosh}
        computers which, when pressed, causes a command line prompt to
        appear.  This gives access to the built-in mini-{debugger},
        which has commands to dump memory, return to the application
        that was broken out, and others.  A more sophisticated
        debugger must be installed in order to inspect {breakpoints},
        etc.
     
        (2000-11-01)