I had noticed the same thing myself too, but as I'd been messing around with external power supplies and switches, thought I had just blown...
I've managed to use the +5V_RUN (pin 4 on CN-15 SATA power connector) to switch a relay on/off to control the ATX power ON (just grabbed spare...
I found the documentation in the end, it is here for anyone looking. https://www.udoo.org/docs-bolt/BIOS_UEFI_and_Tools/EAPI.html
The Bolt manual talks about the Embedded Controller (not Arduino) making additional GPIOs available, but I couldn't see any details anywhere on...
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