Concept for e-Nable prosthetic hands to have more force for weaker wrists that also have a limited bending/travel range. This is made possible with a pair of pulleys combined with a ratcheting mechanism. This would allow gradual increase of the grip of the prosthetic fingers to increase as you bend the wrist several times.

This will act like two pulleys on one axle. Each pulley has a different diameter. If you put cable across both of them, each pointing in a different direction, you can pull one of them and get more strength on the one that has the smaller hub.

The cables should fit through the holes if they're 0.4 mm, but you may want to increase its size a bit if your cables are bigger.

Compared to directly pulling the cables, this will duplicate the force if you make the same amount of bending movement twice. You can also decide on a 1:3 ratio for 3 times the strength. In any case, if you can only bend a little, you can do it three times and still grip completely. 

The pink part would be part of the fixed part of the prosthetic the pulleys that rotate within it need a 3mm bolt as an axle to rotate on, which should also be a fixed part.

A release mechanism hasn't been developed yet, but that shouldn't be too hard. Also, it needs to be tested and the spacings/sizes adjusted. Feel free to continue where I left it.

Updates:

V1.1 now it has animation and also the cables are rendered. You see the cables move with a ratio that corresponds with the pulleys. The force ratio is inverted (slow moving cable has amplified force)

V1.2 made the part a bit bigger, made the teeth deeper and the pawls thinner and longer. Made the ratchet well a bit fatter (stronger) and the holes for the cables wider.