When sewing machines in factories were powered by central steam engines and transmission belts, they were particularly difficult to operate since, unlike a machine driven by a treadle or electric motor, there were only two speeds: stopped and full on. The operator had to be absolutely sure everything was lined up and ready to go before engaging the clutch that drove the machine. One could, with the clutch disengaged, turn the machine manually, but that was the only alternative to full speed.
I watched that last night before seeing your post. It’s probably weird that it made me think of this song