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.
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.