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https://philosophy.williams.edu/files/Egan-Learning-to-Be-Me...

>This method depends on what’s called “referred sensation,” where stimulation of your body in one place gets felt in another place—it’s like when you accidentally bash your elbow against something but can feel tingling up through your fingers, because the signal has traveled along your nerves up through your hand. In some places, including in the fingers, referred sensation can be targeted with a reasonable amount of precision. With a signal electrode on the top of a finger and a ground electrode closer to the wrist, it’s possible to stimulate individual parts of each finger, creating 11 separately controllable tactile zones across five fingers and the palm.

>However, just zapping fingers doesn’t provide a way of getting those signals to show up just on the bottom (palmar) side of the fingers, rather than the top (dorsal) side, which is where the signal is originating. Fortunately, the asymmetrical way that the nerves in our hands are set up makes this possible. The backhanded stimulation technique works because your palm and finger pads are way, way more sensitive than the backs of your hands, thanks to about 60 times more mechanoreceptors on the palmar side. So, if you use an electrode to stimulate the back of one of your fingers, the sensitivity on the front is so much higher that you’re going to feel it there much more strongly, even though the electrode is in direct contact with the back side. The researchers were able to find a stimulation intensity that was enough to trigger nerves on the finger pads, while staying below the detection threshold on the back side of the fingers, neatly solving the problem.