There may be another explanation for the curing of cancer during a fast aside from (or in addition to) the body "eating" the tumor.
In reading up on things, I came across some research where it's believed that a tumor - initially - is part of a healing reaction using a certain type of fibroblastic cells. When the healing is done, they're turned off by enzymes (provided by the pancreas) and that's the end of it.
However, with the modern diet of over-processed foods, pancreatic enzymes are consumed in the digestion of the crap food and the fibroblastic cell growths are never turned off and grow into tumors. So this leads me to wonder if, during a fast, the pancreas is freed of the burden of producing enzymes to digest foods and handle sugars, can dedicate enzymes to other body processes (the human body only produces a finite number of enzymes a day - when it's gone, it's gone) like turning off these cellular activities and rejuvenating damaged tissue.
Just a thought. In any case, temporarily relieving the body of the burden of digestion truly does allow the body to play catch-up. I believe the pancreas and the liver are the stars of the show in this endeavor.