Apple has updated a support document which shows the operation of the dual video card of the MacBook Pro 15 "and 17" of 2010 and later. If you are using external displays or old PowerPC applications, the system performs the switch to more powerful video card. These models include a feature in the Energy Saver preference that lets you set graphics performance.

The option "Auto Graphic" (active by default) allows the computer to switch automatically to the best graphics system based on running applications by selecting the Intel graphics processor for everyday and AMD Radeon for the most demanding graphics applications.

This option also allows you to optimize battery life. If you want to use always the fastest graphics processor, you must deselect this option. Apple is aware that in some circumstances will always use the high-performance graphics system, when the computer is connected to an external monitor (until the monitor is unplugged) and also when using Rosetta, the software "transparent" that up to Mac OS X 10.6.x allows you to run older PowerPC applications (the use of high-performance graphics system will remain in effect until you exit all PowerPC applications).

If you are curious to know which graphics system on your MacBook Pro is using at any given time or enable the system to your specific choices you can use the software gfxCardStatus application (however is not recommended by Apple).

More information - [Apple Support]