Presumably by giving the signal that "food is available" and the system can break out of the hypocaloric energy preservation state. The problem is that the fat deposits are extremely valuable (in a non overweight person) and as one enters a hypocaloric state they become more valuable. I.e your body will rather throttle down metabolism and energy levels farther than start breaking down the fat for energy. Eating some carbs can healp to break this pattern.
I don't think the body works like that. After you eat carbs, your insulin level goes up and the cells get the glucose in them. If there's more than they can take, it is put into the adipose tissue as fat. Eating carbs once every 3 days is in no way telling the body that there is enough food, quite the contrary actually (all this is just my opinion).
This carb refeed reset "tricks" your leptide hormones to burning through energy reserves rather than putting the body through a catatonic (muscle burning) state. This is why the "Cheat" meal or day is an important step to sustaining body fat loss through elongated periods.