Spin in an office chair with your arms & legs sticking out, then pull your limbs in tight to spin faster. If you watched Vancouver’s Spring Olympics, you saw figure skaters slow down a spin by extending a leg, then speed up by simply withdrawing the leg. This has to do with the moment of inertia — the mass distribution impacts how an object will rotate. When more mass is farther out, things spin slower than when the same mass is closer to the axis of rotation.
When really big subduction earthquakes happen, a thick, heavy chunk of the ocean crust pulls in closer to the center of the planet. This redistribution of mass makes the Earth turn a little faster. After the Chile quake, our days are about 1.26 microseconds shorter than they used to be. This is a permanent change to our global moment of inertia.
But megaquakes aren’t the only impact on the length of a day — the moon provides a gravitational yank to slow us down. Over time, the moon is slowing the Earth through tidal friction, while simultaneously the moon is getting sped up by the Earth (conservation of momentum!), thus moving to a slightly higher orbit. Given billions of years, eventually days and months will be the same length, with the same side of the Earth always facing the same side of the moon. More details on this by the Bad Astronomer.