> Whereas other manufacturers keep dedicated GPU framebuffers powered up even when using integrated graphics (because the display is connected to the GPU), Apple connects the display to a custom multiplexer so the dedicated GPU can be completely powered down. I'm pretty sure it's the other way around: the IGPU does the displaying and the DGPU just does 3D acceleration. Other manufacturers don't have a hardware mux because hardware muxes are expensive, and instead have driver support for handing over rendered frames from the DGPU to the IGPU for display. Indeed, NVIDIA has actually made GPUs with no display hardware on them at all, intended for exactly this application.