Gosh, I didn't know that either. When I was quite young I was helping as we baled hay. Got the job of driving a little Ford tractor that pulled the big rope to lift the hay bales up into the big red barn. The men stacking hay in the hay loft were working on filling the far end of the barn, which meant I had a long trip pulling that rope - all the way across our small yard, across the gravel road, and starting down the ditch---I was scared I'd lose control and wouldn't be able to stop the tractor when the signal to trip the rope would be given -- I turned the key off the tractor and waited for a grown up to come to my rescue!