By default all apps will run on the more energy efficient igpu and the OS can choose to switch to the dgpu when more gpu-power is necessary, trading battery time for graphics performance. On most laptops the default gpu can be changed to the dgpu so that everything will always run on the dgpu.
Linux support for switchable graphics currently is not very good. E.g. on many laptops some of the external connectors are only connected to the dgpu and to be able to use those external connectors without issues users need to change the default gpu to the dgpu, resulting in a hot running laptop and the battery draining much faster.
So I will be working on improving switchable graphics support under Linux, allowing the igpu to be used as default, allowing maximum batter life, while keeping everthing working normally. Specifically the following should all work: external outputs, suspend / resume (including suspend/resume with external monitors connected / while docked) and suspending the dgpu when not used.
Some fixes for this have already landed in Fedora 24, if you've a laptop with switchable graphics, please give Fedora 24 a try. Make sure you've all updates installed, and that you're running a 4.6.x kernel (from updates-testing). Then everything should just work, specifically recently I've fixed several issues with external connectors which are only connected to the dgpu, including the dgpu not suspending after a monitor is unplugged from the external connector.
If you're still having issues using Fedora 24 (with the default open source drivers) on a laptop with switchable graphics, please file a bug and put me in the Cc, or if you already have a bug open, just put me in the Cc.
One known issue is that plymouth (the boot splash screen) in Fedora 24 does not Work well with switchable-graphics, in some cases you may
get a black screen instead of the boot splash until the graphical login manager shows. This is esp. a problem if you've a crypted disk, because the dialog asking for the disk-crypt password will not show. You can press ESC to drop to text mode as a workaround. This is fixed in the Rawhide / Fedora 25 plymouth packages, but the changes are too invasive for an update. An alternative workaround is manually installing the F25 plymouth on F24.
A secondary goal of my work on this is to allow people to run graphically demanding programs on the dgpu by starting them with "DRI_PRIME=1 program", note that since we do not support dynamic reclocking of nvidia GPUs this will not always result in a performance improvement. Again if you're using this and it does not work properly, please file a bug and put me in the Cc.