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

grind crank
 n., //

    A mythical accessory to a terminal. A crank on the side of a monitor, which
    when operated makes a zizzing noise and causes the computer to run faster.
    Usually one does not refer to a grind crank out loud, but merely makes the
    appropriate gesture and noise. See grind.

    Historical note: At least one real machine actually had a grind crank ? the
    R1, a research machine built toward the end of the days of the great vacuum
    tube computers, in 1959. R1 (also known as ?The Rice Institute Computer?
    (TRIC) and later as ?The Rice University Computer? (TRUC)) had a
    single-step/free-run switch for use when debugging programs. Since
    single-stepping through a large program was rather tedious, there was also
    a crank with a cam and gear arrangement that repeatedly pushed the
    single-step button. This allowed one to ?crank? through a lot of code, then
    slow down to single-step for a bit when you got near the code of interest,
    poke at some registers using the console typewriter, and then keep on
    cranking. See http://www.cs.rice.edu/History/R1/.


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

grind crank

   A mythical accessory to a terminal.  A crank on the side of
   a monitor, which when operated makes a zizzing noise and
   causes the computer to run faster.  Usually one does not refer
   to a grind crank out loud, but merely makes the appropriate
   gesture and noise.  See grind.

   Historical note: At least one real machine actually had a
   grind crank - the R1, a research machine built toward the
   end of the days of the great vacuum tube computers, in 1959.
   R1 (also known as "The Rice Institute Computer" (TRIC) and
   later as "The Rice University Computer" (TRUC)) had a
   single-step/free-run switch for use when debugging programs.
   Since single-stepping through a large program was rather
   tedious, there was also a crank with a cam and gear
   arrangement that repeatedly pushed the single-step button.
   This allowed one to "crank" through a lot of code, then slow
   down to single-step for a bit when you got near the code of
   interest, poke at some registers using the console typewriter,
   and then keep on cranking.

   [Jargon File]