Display Data Channel... it's the signal that your video card uses to communicate with yours monitors to do things like adjust brightness.
Or it does but the state of DDC/CI (and HDMI-CEC) is very sad as implementation is inconsistent and subpar to say the least (from order of commands to timing to subtle difference in required payloads that would make the display reject commands).
A dedicated device would cut the GPU out of the equation, but most likely patching ddc tools can be equally fruitful.