It's not necessarily a zero-carb diet. I think one of his clients even did APD+Potatoes because it made him happy. IIRC, keeping it under 150g carbs/day will let you hang on to ketosis, though I have trouble even hitting 50g/d unless I make myself eat them. I honestly forget the carb policy on PSMF days, but I'd assume they're actually as close to zero carb as shakes will allow. He's got like 9 blog posts on the diet (Evolution of My Diet, Introducing, FF, NTFF, Italians, Rampage, etc). If you haven't read them all, do so.