https://www.flickr.com/photos/21624415@N04/15715733237/
Graphical representation, albeit crude, of my take on it, and attempts at explanations of a couple years back, as it relates to the concept of power smoothing. Strain energy as an aid to performance has been well known in some arenas for a long time. I think the difficulty as it relates to bicycles is the non-obvious, non-intuitive flexural modes that could provide the benefit though there was some FEA work a while back that demonstrated them. It has nothing to do with getting more energy out of the system than is applied to it, but everything to do with how the power is generated (affecting peak muscle fiber stress and fatigue) and how the power is applied (smoother power application equalling higher propulsion efficiencies). I think its pretty interesting stuff.