The project consists of a shield you can add to an Arduino equipped with another Altmega 168/328 chip and a whole ‘nother set of digital and analog I/O pins.

Instructable user johndavid400 explains:

The shield design can use the base Arduino’s power supply or it can supply power to the base Arduino via it’s onboard power-terminals and 5v regulator. You can select which power mode you want to use via the on-board jumper selectors.

Using I2C, you can connect 1 Arduino (master) to a host of slave Arduino’s through Analog ports 4 and 5.

Also, you can add the security of having a completely separate CPU to your project that is unaffected by any code running on the base Arduino. I use this shield as a failsafe on my R/C lawnmower. The base processor uses the 2 external interrupts to sample and decode 2 servo signals from an R/C transmitter/receiver, while a 3rd servo signal is sent to the Core2duino that controls a relay for the motor-controller power supply. This way, even if there is a problem with the main code and it stops responding, the Core2duino will still be able to carry out it’s main function unaffected (which is to kill the power to the bot if anything unusual happens).

[via Embedded Projects]