The Intel HD 4000 and the Radeon HD 7670m work independently of each other. It's basically done to conserve power / extend the battery life.
By default the laptop will be using the Intel HD 4000. When AMD's or Intel's drivers detects a launched application that can take advantage of the HD 7670m's graphics capabilities (like a game) it will then be the active graphic chip. For nVidia cards/chips this switching technology is called Optimus. Don't know what AMD calls it.
If for any reason a game launches and the laptop does not switch over to the nVidia chip/card, then thru nVidia's drivers you can force the nVidia card to become active when that specific game launches. I will assume AMD's graphic drivers will have the same capabilities as well.