I'm mostly curious about what everyone's experience and knowledge is on a non-USB power-up - using the DC jack. In my case the bootloader runs and waits until watchdog time out. However the optiboot source code looks like it is intended to quick start and bypass the waiting in the case of a power-on reset. I understand the circuitry to signal an external reset from the USB port (for the purposes of programming) - that's very slick.
But, I don't see how the power-up via the DC jack is causing an EXTRF (external reset) which triggers the bootloader to wait.