The PCIe bus, the frame buffer memory, the GPU -- it's all shut down rather than switching to a low-power state. Nvidia says this translates to major power savings, extending battery life by up to double. What better way to demonstrate the technology than by yanking a GPU out of a live system, and then placing it back in as if nothing happened?
The ability to switch between integrated and discrete graphics chips has been around for a while, but has required user input. Users have had to manually make the swap, and sometimes the effect isn't instant. Optimus on the other hand, is totally transparent and allows systems to dynamically switch between graphics solutions without any user involvement.