I'm sure Joel Marion will be on here to answer the question in more detail, but off the top of my head...leptin levels have a direct correlation to the amount of fat you have. Meaning, the more fat you have, the more leptin floating around in your blood and vice versa. So basically when someone goes on a hypocaloric, low carb diet and their fat levels drop...their body is basically going into "we are starving mode"...and metabolism drops and fat loss tends to stall. However, in regards to caloric intake...the whole premise behind a carb re-feed is to bring leptin levels back up, which will help to boost metabolism. As for which macronutrient combos stimulate leptin production the most...I'm not sure, but I would have to think P+C. And in terms of when to "carb up"...I know that you should def do it on a training day for either; one all out, eat what the hell you want meal. Or for 8 hours...where you eat "sensibly". And as for frequency, that is HIGHLY individual. I know for leaner individuals...they can get away with more frequent carb re-feeds. But for the most part, most people tend to stick to once per week, usually on the weekend.
Anyways, I hope I was atleast somewhat helpful and more importantly...on the right track..haha. As I said earlier..I am sure Joel will be on this question like white on rice. Tony G