wphamilton

Are you asking me?

I do not believe that frame flex returns energy to forward motion, other than a tiny amount that is due to some very much secondary effects (which I don't want to go into, since it would start a round of "physics arguments").

I would suggest that the movement of the bottom bracket, due to frame flex, can be harmonious with a person's pedal rhythm, and that sometimes feels better (and draw whatever physiological or psychological implications from that, I'm not suggesting anything specific). In particular - and this is speculating - I can see how a particular orientation of the BB might arise, while the cyclist's body has shifted to one side during out-of-the-saddle pedaling, where his downward force is aligned with the plane of rotation of the crank. With no flex, his force would be at an angle to the plane of rotation, and that at the very least will change how that pedal stroke feels. And, possibly, his time-wise distribution of power through the pedal stroke.