From Popular Science:

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.