These units have drag chains on the floor, which advance the load (in this case, silage) to where it is discharged. They are driven by the PTO of the tractor. The web sites I gave you show pictures of manure spreaders, but the principle is the same. And this is a good safety mechanism; if something goes wrong you may need to cut off the scraper floor in a hurry, plus there have been fatal accidents when people service these things and the scraper floor mechanism is on, i.e., getting caught in the drag chains and pulled through the unit.