From what I know, it isn't possible to change the type of PresentMode that Rendy uses without using your own branch and hardcoding the changes.
There are a number of reasons users would want to use something other than FIFO (traditional vsync), some of which might be:
- lower input lag
- benchmarking
- Gsync/Freesync/VRR displays where you're supposed to disable vsync in game and apply it at the driver level
Example of how changing PresentMode currently works: 6bb8c96