I remembered the name, it was "Overlay" seemly it allowed you do things like control the RAMDAC directly and skip the CPU, send arbitrary information to the CRT

I remembered the name, it was "Overlay" seemly it allowed you do things like control the RAMDAC directly and skip the CPU, send arbitrary information to the CRT, and other shenanigans. From what I understood the non-official reason for its removal was to prevent people from using it to circunvent HDMI restrictions in a way that would allow them to duplicate movies without using DRM. The official reason was that it caused driver instabilities and thus Microsoft was against its inclusion in the new driver model for Vista. Why it was popular with CAD, I have no idea.