The theory of hyperbolic discounting asserts that the farther in the future a consequence (positive or negative), the less it matters to us when compared to an immediate consequence. This is why we abandon our diets in the face of a cupcake: the future hot body just pales in comparison to fatty, sugary goodness right now.
This is why putting clear economic incentives on habits can be so effective. Ian Ayers' Carrots and Sticks is an excellent book detailing how making behavioral contracts can be effective for smoking cessation, weight loss, etc.
So, I'm on board with the method. However, I don't quite understand this motivational structure for working out. Here, a gym membership costs more when you miss a workout. Maybe this is just me (I see my gym time as my favorite time of day), but missing a workout already costs you more because your monthly membership dues means that you're paying more per workout when you work out less.