DirectVR is collection of several features ranging from adjusting the game resolution over applying useful tweaks to handling FOV and head tracking. What exactly is available (if at all) and how it is done depends on the game.
These features either ‘just happen’ or you are prompted to do something if necessary (e.g. if the memory scanner has to be run or you have to restart the game because DirectVR changed game settings).
If any DirectVR features are available for a game, there is a DirectVR page in the vorpX menu with relevant options. If there is no DirectVR page in the menu, no DirectVR features are available for this game.