The butterfast is amazingly simple. All you do is eat a stick of butter in the morning (can be melted or nonmelted). For the rest of the day, you can basically eat whatever you want until the butter passes through. It seems completely illogical, but the butter doesnt lie.
I think it coats your stomach or intestines (you know, the part that absorbs the calories?). And the food just passes right on by.
Warning, the butterfast does have a slight laxative effect, but butterfasting is much gentler than laxing.