What exactly does it really mean to go on a low carb diet?

That you eat less: Those no carb diets all work by the same mechanism every diet on earth works by: less calories in. The key is that if you eat enough calories to gain weight but do it from fats and protein alone, it;ll make you sick, so most people end up eating substantially less. Try it and see (myfitnesspal is a great app for this). Every 3500 cal missed (or so), another pound will fall off.