It does it automatically based on usage; things like regular use (browsing, working on documents, etc) will use the low power Intel graphics, while games and such will use the faster Nvidia graphics.
As for as changing it manually, I haven't seen yet how or if it is possible on those setups. It also pretty much means you can only use the drivers
for each from the laptop manufacturer as they will have modded the install/setup routines of the drivers to work with this switching. Another thing from what I have seen with these laptops, is that the Intel driver
has to be installed first or the other (doesn't seem to matter if it is Intel with Nvidia, or Intel with ATI) driver will not install.