How does the GPUs (Nvidia and Intel) work in Alienware 17r1? When the Nvidia GPU begin to work it?

Hello

I want to know how works the GPU´s in alienware 17r1. Does Intel GPU and NVIDIA GPUs works together in same time or only in applications that it no require  graphic processor power use the GPU intel and  it use  the GPU NVIDIA when a application that demand more power? or Alienware 17r1 used only one at the time, all the time?

Regards

Jose Antonio.