I also see you have an enormous capacitor after the diode on the 12v side of your circuit - why so big?
Ah yes, that's a bit of a trick. It's explained in detail in the book, but the short summary is that it holds the output high for long enough for the Arduino to go through a managed power-down routine such as closing the logfile on the USB memory stick. It has one of the interrupts connected to a voltage-divider (protected by a Zener clamp) on the input side of the diode you mentioned which is there prevent the input-side cap discharging back into the loom when the power goes away, and the ISR forces the main loop to bail on whatever it's doing and clean up after itself.
Jon