John,
I am fairly sure the one we had on a model 62 combine had a clutch.  I remember that to engage it was necessary to push down a lever that protruded out from the bell housing.  You then released a lock tab and let the lever up slow to engage the clutch and start the whole machine turning.  There was a rope from the end of the lever under a pully. and to the tractor seat.  In the event of a plug or a stone in the combine, pulling the rope disengaged the clutch drive to minimize damage to the machine.
 I am not sure about the unit on the New Holland baler. That may have been belt tighten as you suggest. I will look at that unit the next time I am out back.
Charlie