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rive_gauche
So further reading and judicious use of my degree certificate says they work in the same way as your touchscreen on your phone. They measure the change in the permittivity of the material around you. You do that by having (crudely) a parallel plate capacitor. One side is the detector and the other side if the ground. You're actually detect (mostly) the water content of the wood as it behaves very differently to free space (air).