It allows you to multiply the food intake that you have, as long as you are also drinking a Coke Zero at the same time.
so for example, say you want 3 double cheeseburgers, at 1000 calories a pop. that's 3x1000x0(the calories from the Coke Zero). That is always going to equal 0. That's 0 overall calorie intake.
For that same reason you should steer clear of Dr Pepper 10.
This is the trick that fad dieters don't want you to know about. That's why I always sneak in a Coke Zero to Chuckarama.