Basically it uses the villager mechanics, similar to breeding, where the farmer will give food to the other villagers. What you do here is build a farm with a farmer villager. Then you take a second villager and stick him in a little box off to the side behind a hopper. One of the problems with villager farming is that they fill up, even if you give them seeds. But in this scenario, the farmer goes over and throws food at the other villager in the box, but it gets picked up by the hopper instead and funnels down to your chest. The villager essentially dumps the food into the chest for you and he doesn't fill up.
The one thing that I also learned (for those that don't know) is that you need to put your farms 65-ish+ blocks away from any villages. I have a villager breeder and originally built these farms within that distance and the farmers just stand by the fence and stare toward the village. After moving the farm, the farmer instantly started picking potatoes and throwing them at the other villager.
Video:
https://www.youtube.com/watch?v=eRlfB5lx1oc
Screenshot of one of my farms:
