I hear responsiveness used often on shoe reviews, yet I'm not exactly sure what it means. Is it about firmness of the shoe, with firmer shoes being more responsive than soft shoes? Is it about flexibility? Is it about how fast the sole flexes back when you try to bend it? Is it about the bounciness of the shoes? Is it synonymous to "energy return"? Does more cushioning entail less responsiveness? Is a responsive shoe ideal for racing?