The main concept of this is easy enough to grasp, the movement is simulated by pulling against the springs. I saw this (https://www.instructables.com/id/Mechanical-Hand-2/) a while ago and really liked it so I made an attempt at it and ended up incorporating more degrees of freedom.
I've included four gifs showing movement, Pinky and ring finger, thumb curling inwards, pointer and middle finger and fist.
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