"PCI Express provides a mechanism for software controlled limiting of the maximum power per slot
that a PCI Express adapter (associated with that slot) can consume."
" Scenario 3: An Adapter Consuming 30 W
If the adapter is plugged into a form factor #1 40 W slot, the Slot Power Limit control
mechanism is followed, and the device operates normally.
If the adapter is plugged into a form factor #1 25 W slot, the Slot Power Limit control
mechanism is followed, and the device must scale down to 25 W or disable operation.
If the adapter is plugged into a form factor #2 15 W slot, the Slot Power Limit control
mechanism is followed, and the adapter must scale down to 15 W or disable operation. An
adapter that does not scale within any of the power limits for a given form factor will always be
disabled in that form factor and should not be used. "
I dont know how the motherboard 'disables' the adapter/video card in this scenario, I assume the motherboard disables it by cutting power if it doesn't want to scale down but I haven't read it completely though.
There are processes through which the chips that govern power supply to the GPU interface with the motherboard to tell it how much power needs to be delivered. There's a really long section about it in that whitepaper and it lists all the commands and protocols that get sent by the board and by the GPU to tell the board how much power to allocate on POST and once the system passes POST.
It's quite complicated, I'll admit. I started hitting PG DN once it was clear what I was reading. My eyes needed a break.