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A bicycle's crankshaft drives the chain of your bike as you pedal. Most bicycles come equipped with either a double or triple crankshaft. A triple crankshaft gives you a greater lower range of gears, allowing you to pedal uphill.
Compact crankshafts are not new, but are just coming into popularity. They also give you a better lower range, but take away a bit at the higher range, unlike the double or triple crankshaft. The upside is that it is lighter weight than even a standard double. The downside is that you don't get as great a lower end as a triple would give you and you do lose that bit at the high end.
So, if you need better low end gears on your racer, but don't need all that a triple would provide, a lighter, compact crankshaft could be your answer.