The default setting will auto switch between integrated graphics and discrete graphics base on whether AC adapter is connected. With AC adapter, it will run in discrete graphics mode; on the other hand, it will auto adjust to integrated graphics mode while running without AC adapter.
 
If you want to adjust manually, setting is available in ATI Catalyst Control Center. Please right click on Windows desktop > [Configure Switchable Graphics] and select [High-performance GPU] or [Power-saving GPU] base on your need.