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

twirling baton
 n.

    [PLATO] The overstrike sequence -/|\-/|\- which produces an animated
    twirling baton. If you output it with a single backspace between
    characters, the baton spins in place. If you output the sequence BS SP
    between characters, the baton spins from left to right. If you output BS SP
    BS BS between characters, the baton spins from right to left. This is also
    occasionally called a twiddle prompt.

    The twirling baton was a popular component of animated signature files on
    the pioneering PLATO educational timesharing system. The archie Internet
    service is perhaps the best-known baton program today; it uses the twirling
    baton as an idler indicating that the program is working on a query. The
    twirling baton is also used as a boot progress indicator on several BSD
    variants of Unix; if it stops, you're probably going to have a long and
    trying day.


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

twirling baton

    The overstrike sequence -/|\-/|\- which produces an
   animated twirling baton.  If you output it with a single
   backspace between characters, the baton spins in place.  If
   you output the sequence BS SP between characters, the baton
   spins from left to right.  If you output BS SP BS BS between
   characters, the batton spins from right to left.

   The twirling baton was a popular component of animated
   signature files on the pioneering PLATO educational
   time-sharing system.  The "archie" Internet service is
   perhaps the best-known baton program today; it uses the
   twirling baton as an idler indicating that the program is
   working on a query.

   [Jargon File]

   (1995-02-23)