OK. I'm trying to picture this.
Pedals wind up a spring, say like a spiral clock/watch spring. Pedaling effort would be somewhat easier if the spring is close to unwound and higher when the spring is close to completely wound. Seems like you could pedal at any cadence you want, no?
Spring powers the rear wheel. A transmission would need to go between the spring output and rear wheel to match the spring force to the required driving force depending on speed, incline, etc.., right? How would you "throttle" the power? Perhaps you could shift to high gear to reduce driving force or to a low gear to increase it, so the transmission would serve as a kind of throttle.
Sounds like an interesting novelty, but not sure of the real benefit or commercial potential.