To make a game run using the discrete GPU, use these simple steps:

1. Select a game - that you want to run using your discrete Nvidia card - from the Library page of the Steam client, right-click, and select Properties.
2. Click the SET LAUNCH OPTIONS... button and specify primusrun %command% for the command line.
3. Save your changes.This method allows you to pick when the discrete NVidia GPU should be used on a per-game basis.