Without knowing what laptop your talking about........
Most of the new ones use the integrated graphics for general use, then kick in the discrete graphics when it's needed, this saves power and keeps it running cooler.
When you play a game, the Nvidia graphics will kick in automatically if it's needed.
sorry for not mentioning it i completely forgot abt it....
its lenovo ideapad z570 and the graphics card is nvidia geforce 420gtm
edit: sorry not 420 but 540gtm