The way it was described to me is you expend a fixed amount of energy to get from point A to point B. Cadence determines how much of that energy is supplied by the legs and how much is supplied by the cardio system. High cadences mean high cardio outputs with lower leg outputs, and mashing means high leg outputs with low cardio outputs. The advantage to high cadences is that your cardio system will last a lot longer than your leg systems, and once cardio is kaput you still have your legs to draw upon; if you burn through your legs first, the best cardio system in the world can't come to the rescue.