Laptop with integrated & discrete cards combination are designed to work alternately depending on the workload. If you are just using for normal productivity or browsing, the integrated will take over to save battery life. But when you start putting heavy load on graphics, the 7970m should take over. It should be automatic & user does not need to interfere.
If you use AMD discrete graphics, it is called Dynamic Switchable Graphics.
If you use the nVidia version, it is called the Optimus Technology.
Here is good reading:http://www.notebookcheck.net/AMD-Dynamic-Switchable-Graphics-vs-Nvidia-Optimus.64378.0.html