This has to do with the filter/codec used for display.
Check under
(right click >) Options > Filters > Advanced (also click the [+]-s).
Check if video-filter/codec has option to "flip screen" or "flip video" or something similar in its properties.
PS: I don't think you need to use BS.Player's "extend-feature" for your purpose.
(
Graphical explanation of "extend"-feature)
[Depends on your Nvidia's properties-settings]