Maybe not cognitive, but there is a switch. Everything about your metabolism and everything your body does is controlled by enzymes and receptors wich are the product of genes. The reason Atkins induction works is because in those two weeks or more when you are starting to consume much more fat than carbs your body realizes that carbs are not around anymore. This is realized by receptors and other cellular machinery in very complex, but very real ways, and your body does make a metabolic switch to a fat burning metabolism.

When fat is the main calorie source, your cells have to "upregulate" fat burning enzymes at the genetic level. This means that your body transcribes the genes that control fat catabolism a ton more than they did before. For this reason, people on low carb diets are much better at mobilizing fat in all circumstances.

I don't know for sure that LCers will burn body fat rather than body protein in a fasted state, but they absolutely have much more capacity to do so than someone who does not restrict their carbs.

Jamian-B.S. Biochemistry and Molecular Biology, UC Davis