It works — but so does daily calorie restriction, and at matched calories the cleanest trial found fasting lost less fat. One good 12-month study does favour it, for a reason nobody sells: people stuck to it better.
Protein genuinely preserves lean mass while you diet — replicated across four meta-analyses. But the effect is small, it mostly fails without resistance training, and the metabolism claim was dismissed by a review the meat and dairy industries paid for.
Don't eat protein with starch — the rule has been sold since 1929 and tested exactly once. In that trial the food-combining group lost slightly less weight. The digestion claim has never been tested at all.