Tablets are usually either pressure sensitive touchscreens, or as I suspect in this case use digitizers (Wacom or other tablets) that transmit an RF signal across the back and pick up the resonance (sort of like RFID tags or the security devices to prevent shoplifting, but with XY precision). So the stylus has an inductor and some other electronics.
This (or a close variant) is probably what you are looking for:
