They store the energy mechanically by rotating mass around an axis.
它们通过质量绕轴旋转机械存储能量。
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An earthquake can make earth rotate faster by nudging some of its mass closer to the planet's axis, just as ice skaters can speed up their spins by pulling in their arms.
Since that pulls mass closer to the planet's spin axis, it causes the planet to rotate faster - just as an ice skater spins faster when she pulls her arms towards her body.