Making hay with horses
In the hayfield, the hay sweep (or sweep-rake) moved hay. The driver lowered the sweep's teeth (the long wooden tines) to ground level and took the sweep down a windrow of raked hay. As the sweep moved forward, the hay piled onto the buck (the platform of wooden teeth.)
When the buck was full of hay, the load was taken to the haystack. A good man on the sweep planned his route so he was close to the haystack when the buck was full and heavy.
Mowing machines and dump rake were pulled from the front. However, horses could not walk in front of the sweep-rake -- they would have been wallowing through the hay windrows that the sweep was supposed to gather. This problem was solved by having the horses pull the sweep from the sides.
In the image below, the load of hay has been deposited on the haystacker buck (another platform of wooden tines.) The sweep driver has backed the horses away from the load. Now he is approaching the hay again to push it farther onto the stacker buck. The driver is physically lifting the sweep teeth. Men and horses worked hard in the hayfield.
By the time I was a child, the hay sweep was a tractor, with its transmission reversed and its seat turned around. Its big wheels were in front and its small wheels were in back. The sweep buck was mounted in front of the big wheels, and it was raised and lowered with hydraulic power. It could carry much larger loads of hay than a horse-powered sweep, and it went much faster.
A tractor pulled the load of hay to the top of the haystack. However, the tractor driver was still said to be "driving the stacker team." Maybe "stacker team" was easier to say than "stacker tractor."
Farm Security Agency.
Horse-drawn Hay Rake
Solomon Butcher's Nebraska images of hay equipment (stacking)