The graphite rectangle drawn on the paper conducts electricity, but has quite a high resistance. As it bends, the resistance either increases or decreases, depending on which way it is distorted. By measuring that resistance, the precise angle of bend can be detected.
By gluing the paper to a finger, the researchers showed that it could monitor the movement of limbs. They also attached it to a ruler and showed that it could measure how bent the item was – a proxy for determining the pressure exerted on the ruler. And inside the fold of a book, it could measure the angle to which the book was opened.
via Pencil sketches turn paper into a sensor – tech – 18 March 2015 – New Scientist.