What mechanical link allowed inventors to decouple speed from wheel diameter?

The chain drive connected to the rear wheel

The fundamental engineering problem plaguing earlier designs like the Penny-Farthing was that achieving speed necessitated an enormous driving wheel because the pedals were directly linked to that wheel. The crucial breakthrough was decoupling speed potential from wheel size, which was achieved by introducing the chain drive mechanism connected to the rear wheel. This allowed a small sprocket near the pedals to drive a larger sprocket on the rear wheel via a chain. This gearing system provided mechanical advantage, meaning the rider could achieve significant distance covered per pedal stroke without requiring an impossibly large wheel, thereby fundamentally solving the speed/size paradox.