Nvidia Optimus Technology Explained

 All this to save battery life...I would think that anyone who buys a "gaming laptop" usually runs plugged into AC
 when gaming ???

 IMHO, very complicated and a problem looking for a place to happen...

Nvidia’s Optimus technology is a great thing to have on your laptop, especially if it uses a more powerful discrete graphics chip like a gaming laptop normally would. The idea behind the Optimus tech is that it can automatically switch on and off the more powerful discrete GPU, depending on the current usage of the laptop, for example if you run a game the discrete video card becomes activated and you can enjoy high fps. But when you are working on the laptop with office applications or browsing the web you don’t need a powerful GPU and the integrated Intel GPU can do just fine, so the discrete GPU gets completely shut off saving battery. This way you get the best of both worlds – high performance in games and other demanding applications and longer battery life when you are on the go and need longer battery life.

The implementation Nvidia uses has the integrated Intel graphics active all the time, no matter if it is the GPU rendering the image, or only visualizing what is rendered from the discrete Nvidia GPU. When you run a game, the more powerful Nvidia video card gets activated and starts rendering the frames and as soon as a frame is being rendered it is sent to the integrated GPU’s framebuffer for visualization. So the integrated Intel graphics actually shows the image on the laptop’s screen no matter if it rendered the visual data or not, and so all the extra video connectors, such as HDMI for example, are usually connected to the integrated Intel GPU and not to the discrete Nvidia one.

Source : http://3dvision-blog.com/5532-nvidia-optimus-technology-and-3d-vision-dont-go-well-together/