The Organization's schematic is incomplete, but it does show
GLOBAL_EN connected to U2 - the PMIC (Power Management Integrated Circuit). Pulling
GLOBAL_EN low (aka shorting to GND) resets the PMIC - which causes it to power up all its outputs, in turn causing the Pi 4 to boot - it's exactly the same as when you disconnect power, then re-connect it.
Your question is not completely clear to me, but when you say, "I am not sure exactly how" you may be asking, "How would one implement this feature - shorting
GLOBAL_EN to GND?" If that's part of your question, there are several ways this could be done, but I know of nothing in the current Pi 4 hardware that would support this. One typical way this is done on other machines is with a RTC (Real Time Clock), but Pis have never had one. Another way would be with an external sensor of some type that provided a switch closure when some condition was sensed. Still another way would be to use a small microcontroller (Arduino for example) to set the state of
The documentation states:
If 1 and WAKE_ON_GPIO=0 then switch off all PMIC outputs in halt. This is lowest possible power state for halt but may cause problems with some HATs because 5V will still be on. GLOBAL_EN must be shorted to ground to boot.
If you've not seen it, you might want to read this answer, or if you want full details try this GitHub page.. Now we've both confirmed shorting
GLOBAL_EN to GND will in fact boot the RPi 4 from
shutdown when the EEPROM firmware is configured iaw the documentation.