I read an article about this years ago and came away with the impression that they were effectively mummifying themselves before death. From the Wikipedia page on sokushinbutsu:

In medieval Japan, this tradition developed a process for sokushinbutsu, which a monk completed over about 3,000 days. It involved a strict diet called mokujiki (literally, 'eating a tree'). The monk abstained from any cereals and relied on pine needles, resins, and seeds found in the mountains, which would eliminate all fat in the body. Increasing rates of fasting and meditation would lead to starvation. The monks would slowly reduce then stop liquid intake, thus dehydrating the body and shrinking all organs.

https://en.wikipedia.org/wiki/Sokushinbutsu