Our neighbours were stacking their hay this way. The pole is set up leaning slightly into the stack, the man on the ground has a grab which can dig into a biggish lump of hay, and the horse pulling on the rope, passing around three pulley blocks, hauls it up. The man on the stack can swing it around an arc, before the man on the ground jerks his rope which releases a catch and drops the hay. Then still pulling, he rotates the gantry so that, as the horse comes back, the grab drops clear. There is a boy leading the horse, but they told me that an experienced horse could do it alone.