"""a whole lot of people try this but go back to eating junky food after they fast"""
I think the theory is that the fast cleans out the junk in the body that keeps the person addicted to those foods.
In theory its true.
If all that was cleaned out of the body, there wouldn't be any cravings for it.
But it isn't done during one fast.
Some people falsely believe that a whole lifetime of bad eating can be corrected in one shot.
So I think that's why it doesn't appear to work.
I think you will see that the people that have success with getting cleaned out through fasting are the ones that do a fast and start to change the diet after the fast.
Then periodically fast and continue to tweak the diet towards something more optimal following each fast.
Its really an ongoing project and the ones that stick with it ultimately experience a healthy body and lifestyle.