So I made my FauxNext out of paper with a dozen layers of clear tape over it to simulate the stiffness of a Flex device.
I tried it on the back of my hand: it’ll work, and it doesn’t even feel that awkward at all. But the positioning needs to be surprisingly precise: too far forward and it’ll bend with the knuckles. Too far backwards and it’ll bend with the wrist. And since it’s already slightly bend left-to-right due to the curvature of the back of the hand, it’s not going to end well if it gets bent in the other direction. Also, I feel it should be contained entirely within the space of the metacarpals - i.e. no overhang on the blade or on the thenar area:
I tried it on my forearm too: it’d work, but good thing I don’t wear a watch 
Also, again, it can’t be too close to the wrist to avoid bending in both directions:
I think the major problem with the back-of-the-hand placement is the risk of collision with something. For the wrist, the risk seems to be that it’s extremely easy to crumple. Hmm…
I’m gonna ponder some more, but I think I’ll end up pulling the ole credit card…