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As you can comprehend from the video, the point of Fusion Engine technology is to combine the work of optic sensor with accelerometer and gyroskope. When you move your mouse very fast, optic sensor could be out of action, and here is where accelerometer and gyroskope come to help, defining mouse speed and acceleration, which allows to anticipate mouse movement.
Such technology gives you an opportunity to increase sample rate up to 1000 Hz, i.e. every millisecond. You can check it yourself by opening mouse settings:
Besides, Logitech software offer us to test how faster you can move your mouse with their technology (description say up to 10 meters in second, speedometer highest value is 14!):
For instance, I wasn't able to do even 4 meters in second, so this techonology applies for VERY fast gamers. Still it's good to know that even in the most critical situation your mouse won't fail you. Quoting engineers, "the user has only to concentrate on his playing style and can't blame anymore his mouse" :)