Popular Science calls the NuVinci the infinitely geared bike:
Cyclists have been waiting a long time for this one. Based on a 1490s sketch by Leonardo da Vinci, The Ride’s rear hub mimics an infinite number of gears, rather than the mere 21 offered by the usual chain-yanking transmission. So you can always find the perfect gear ratio, whether starting from a stop or speeding down a hill. Twist a dial on the handlebar, and ball bearings in the bike’s NuVinci transmission tilt between two rotating metal discs. (Your pedaling turns one disc; the other transfers power to the rear wheel.) As the balls tilt, they touch the discs at varying angles. This changes how fast the wheel spins relative to your pedaling—slowly for low gear ratios, where pedaling is easy but the wheel doesn’t turn much, and quickly for high ratios. The balls can roll to almost any angle, giving you precise control over the bike’s torque (and your exertion). This latest take on da Vinci’s continuous transmission has potential uses beyond bikes. Within four years, expect to see the NuVinci in cars, tractors, even wind turbines — the possibilities are nearly as limitless as the gear ratios. 3,000; ellsworthbikes.com