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I'd assume the accelerometer is able to differentiate between the sharp staccato of a rapping of the knuckles vs. the steady circular motion of rolling.

Where did you get that it mimics the screen?

It has its uses, I usually leave my laptop unlocked when I'm amongst trusted friends, a layer of protection that's this quick is a great idea in my mind. I think it matters on the use case, if you're on a computer where security is paramount, don't use it, but is a convenient feature to have for a lot of other situations.

if you're too excited you might test the durability of your screen!!