The proprietary Nunchuk port speaks the standard I2C protocol, which is probably how they made the MotionPlus pass through extension controllers (as a kid I thought the MotionPlus and Nunchuk used separate sets of pins). I hear all extension communications were encrypted (which is causing problems for Dolphin Nunchuk TAS recordings), but evidently third parties were able to figure it out, since third-party/knockoff Nunchuks appeared fairly quickly (including the Nyko Kama).