According to the caption text: "About 80 percent water, the potatoes lose
much of their moisture before the stomping by natural freeze-drying in
frosty nights and warm days. Soaked in water to dissolve bitterness, then
dried, chuno may keep as long as four years."
I remember more detailed articles on the subject, perhaps reprinted in a
book by Gene Logsden or Wendell Berry.
By the way, in a book whose title and author escape me (maybe something by Wendell Berry?) I read about the author's travels in Peru, in the mountainous areas where the peasants grow wondrously diverse crops in tiny patches of steep mountainside which they keep in place with rocks and nurture with manure. There is a brief mention of watching from a distance as farmers worked at "FREEZE-DRYING potatoes" - and also a description of the delectable finished product. I can understand the drying but how did they do the freezing part? Much too primitive an area for electricity.