Atkins Diet: How exactly does citric acid prevent a person from losing fat? What proof is there of this?

I would like a scientific ysis, quotes from Dr. Atkins (plus source, i.e. books) or references/links to studies. I don't think it's true. If one is not consuming carbohydrates, their body is forced to burn fat. I notice many people on Atkins consume artificial sweeteners even though they're not supposed to...