Low-carb diets work in the sense that the body is producing less Insulin. With less insulin, the body isn't using the food consumed as energy, rather it's body fat. The reason that you go high-fat is that it doesn't cause an insulin response (protein and carbs do)
However, the downsides to keeping this regime in the long term is the lack of muscle glycogen that is used for the workouts. Simply put, the body cannot replenish it fast enough with fat alone. This is where the idea of nutrient cycling comes in where periodic refeeds of "normal" eating is done to replenish muscle glycogen.
One last thing, it's gonna be difficult to get "ripped" using P90x just because of the limitations that you can do using the equipment/program suggested (with respect to the time commitment). As much it helps you get in shape, eventually you may want to decide on learning the "big three" and its many fitness programs that revolve around it (Of course, the big three are the Squat, Bench Press, and Deadlift)