I think after a big treat(?cheat) day the weight balloons up as all the carb/sugar is stored rapidly as glycogen in your muscles (ready for action), but each glycogen molecule hangs onto water molecules thereby increasing ones weight. Once the glycogen is burnt up (when you start locarbing again and using these reserves), not only do you deplete the glycogen but also the water that it hangs onto is released and pee'ed out!...Atleast that was my understanding from DANDR.