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

demo mode
 n.

    1. [Sun] The state of being heads down in order to finish code in time
    for a demo, usually due yesterday.

    2. A mode in which video games sit by themselves running through a portion
    of the game, also known as attract mode. Some serious apps have a demo
    mode they use as a screen saver, or may go through a demo mode on startup
    (for example, the Microsoft Windows opening screen ? which lets you impress
    your neighbors without actually having to put up with Microsloth Windows
    ).


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

demo mode

   (Sun) 1. The state of being heads down in order to finish
   code in time for a demo, usually due yesterday.

   2.  (Or "attract mode") A mode in which video games sit
   by themselves running through a portion of the game.  Some
   serious apps have a demo mode they use as a screen saver, or
   may go through a demo mode on startup (for example, the
   Microsoft Windows opening screen - which lets you impress
   your neighbors without actually having to put up with
   Microsloth Windows).

   [Jargon File]

   (1995-02-22)