Here’s a link that explains it a bit. All I can tell you is that you will get more potatoes if you hill them up, with soil or straw or peat.
http://www.veggiegardener.com/how-to-hill-potatoes-in-a-raised-bed/
If you need more info than this, let me know and I’ll ask Hubby how to explain it tomorrow. He’s already asleep tonight.
I added more of the medium that came with the potatoes in the double pots, to cover the growth of the stalks in them. I get why you do that now. I don’t get how my parents did it but there were lots of potatoes to dig.