Using NVIDIA's Optimus technology, when the discrete GPU is handling all the rendering duties, the final image output to the display is still handled by the Intel integrated graphics processor (IGP). In effect, the IGP is only being used as a simple display controller, resulting in a seamless, flicker-free experience with no need to reboot.
When less critical or less demanding applications are run, the discrete GPU is powered off and the Intel IGP handles both rendering and display calls to conserve power and provide the highest possible battery life.