Every farm in southern Spain has something called an 'era' which is a flat dirt circle, I think called a threshing circle in English, where the hay would be put after being cut with a scythe. A wooden board with rows of knife-like wheels underneath was pulled by a donkey and driven with long-reins by the farmer. Weight must be applied to the board in order to cut the hay, hence the children. There are actually several different boards with different types of knifed wheels for each phase of cutting. It was a very exciting time for the children when the farmer called them to come and sit on the board while he went round and round. It takes several days to cut the hay into small pieces and release the grain from the stalk. It is a sticky job, in the heat you get covered in pieces of hay and it is a bit like a ride at an amusement park, bumping up and down it is a rough ride especially when the hay is in the centre at the beginning, it gets to be a smoother ride as the hay gets spread around the circle. The board sometimes even flips over.
From Barbara Napier's Animo Stories here.