This sounds strange, but the idea is that if you're really going to push yourself on a given day (e.g. doing 2 hours of cardio instead of 1), make sure you overdo your normal water intake and calorie intake too. We tend not to, thinking that the extra exercise will burn off more weight.
But ultimately, all it will do is make you sick - you'll either be burning up lean muscle mass if you're not taking in enough food, and you'll wind up dehyrated (with an IV in your are, like I recently had), if you don't up the water intake. So, if you're going to overdo, overdo it all :-)