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On a regular bicycle, the input force comes from your legs, turning the pedals, which in turn spin the chainring the big toothed gear up front. A chain carries that force to the cogs connected to the hub in back, which in turn turns the rear wheel; the ratio between the number of teeth on the chainring and the number of teeth on the current cog determines what gear youre in. A "high" gear turns a big ring up front and a small cog in back--hard to pedal, but a lot of movement from each revolution. Reverse it and you get a "low" gear--easy to pedal, but not much movement.
But more often than not, there is no right gear for a given situation fourth gear is too low, fifth too high, say. So why not do away with those pesky steps altogether? A continuously variable system does just that.