It's been along time since I ran a belt on a WD Allis, and I can't recall all the steps to run it.
I do recall a lever (dog) down by the clutch pedal and when that was flipped into place the clutch was held in and I believe the belt pully stopped.
That was after the belt pully was put into the run position using the jam nut and bolt that held it in the operate or non-operating position.
When it was physically disengage by sliding it out (I believe) about an inch it wouldn't be powered.
So to answer your question I recall it stopping with the foot clutch.