I may be wrong, but if it is the one i am thinking of, it involves you cutting out all carbohydrates. A book was written about it, the author and founder of the diet however has recently died of a heart attack, not great advertising.
You cut everything except 20gramms of carbohydrate from your diet, and encourage your body to go into a state of "ketosis", where it starts breaking down body fat. On the plus side you lose weight quickly (I lost nearly a stone in a fortnight) on the negative side you can get bad breath, greasy hair and problems with your kidney and heart. My advice would be to research things very thoroughly. I don't do the diet any more.