It’s a small PCB that adapts the Wii Nunchuck connector to standard 4-pin header. It's called the “wiichuck adapter”. It plugs directly into the Arduino, no wiring necessary. If you want to hook up a Wii Nunchuck to an Arduino but don’t want to cut up the cord on your Nunchuck, you gotta have this. Tutorial and Software http://todbot.com/blog/2008/02/18/wiichuck-wii-nunchuck-adapter-available/