A stylus works by allowing the user to easily apply pressure to a specific area of the resistive screen of a device. The pressure brings two layers of the touch screen into contact, activating the microchip.
Resistive systems use a layer of resistive material and a layer of conductive material that are separated by spacers to keep them apart. When a touch device such as a tablet is being used, an electric current runs through both layers of material. When the stylus is used to put pressure on the screen, the two layers of material come into contact with each other, changing the electrical field for those two layers.
A microchip located inside the device interprets the change in the electrical field and translates it into coordinates on the screen. In a tablet, the CPU takes the coordinates from the screen and maps them against the device's operating system. The CPU then determines which application has been activated and launches it for use.
Resistive screens are susceptible to damage if too much pressure is applied to the screen. Excessive pressure may cause the resistive layer and conductive layer to remain in constant contact and the device can't correctly interpret the commands entered on the screen.