It's fiction.
A litre of water weighs one kilogram - and people will vary in weight during the day depending on how much water is in their system - which has led some people to take food or medication which cause the system to shed water (called diuretics), which makes you temporarily lighter on the scales, but doesn't change how much fat or muscle tissue you have, so it's just a way of deluding yourself (unless you are a boxer or a jockey).
Of course, you can get Dianetics over the counter though ;-)
Prescription diuretics are serious drugs used to treat high blood pressure and cogestive cardiac failure etc.Can diuretics be bought over the counter?