[protopete]

In an nVidia Optimus configuration, there is no actual switching between the IGP and GPU. The LCD display is connected only to the IGP. The discrete GPU is connected only to the PCIe bus. Therefore to display anything from the GPU, the contents of the GPU framebuffer RAM need to be copied over the PCIe bus to the main RAM and displayed using the IGP. The Windows drivers use "Microsoft detours" in the Display Driver Interposer to seamlessly direct graphics API calls from applications to the IGP or discrete GPU.

http://www.nvidia.com/object/LO_optimus_whitepapers.html