I can actually answer this (as it is how I stumbled on to the repo), it's through a signal from the motherboard called Pwrbrk (Power Brake), Pin 30 on PCIe. It tells the PCIe device to maintain a low-power mode, in the case of Nvidia GPUs it's about 50W (300Mhz out of 2100Mhz in my case).

You can check if it's active using `nvidia-smi -q | grep Slowdown` as shown in the post