Four spindles on the trackball have a tiny circular magnet the end; each of these are paired with an SMD hall effect sensor, which are used to measure up, down, left and right movements of the trackball.
This is a simple slide switch for the LilyPad. Use it as a simple ON/OFF switch, or to control LEDs, buzzers, sensors, etc.
Need a membrane keypad to surround your LCD? This unit from Matrix orbital is designed to frame any LK/VK 204-25 unit.