The implication being that when a frame flexes such that a rider's full energy is split between forward motion and downward flex, that upon un-flexing the frame returns that energy directly into forward motion. I suspect a spring could achieve this, aside from heat loss, as it's force is in-line with both compression and return. But a frame's flex would have to be spot-on equivalent and in-line, and occur at a time when the rider could actually use it, for the return, in order to recover that energy in the form of the forward motion that was previously lost. Wouldn't it?