AssumeFPS

[Selur]

You must be speaking of something else than I am.

std.AssumeFPS(clip clip[, clip src, int fpsnum, int fpsden=1])

Returns a clip with the framerate changed. This does not in any way modify the frames, only their metadata.

The framerate to assign can either be read from another clip, src, or given as a rational number with fpsnum and fpsden.

It is an error to specify both src and fpsnum.

AssumeFPS overwrites the frame properties _DurationNum and _DurationDen with the frame duration computed from the new frame rate.

source: http://www.vapoursynth.com/doc/functions/assumefps.html
Same in Avisynth:

AssumeFPS

Changes the frame rate without changing the frame count, causing the video to play faster or slower.
...

source: http://avisynth.nl/index.php/FPS


The normal AssumeFPS does not allow "to produce stunning slow-mo" it just changes the playback speed it doesn't change the frame count.
It's exactly the same as the two option I mentioned.
So if you take a 29.97fps interlaced source, bob it to 59.94fps and interpolate it to 89.91fps and then use AssumeFPS(29.97) it's just changing the playback speed and the same as if simply changing the fps flag in the file.

-> So atm. I see nothing special that AssumeFPS does.
Best you share a script which shows where you think something 'stunning' is created through AssumeFPS, because atm. I really don't see anything special.

Cu Selur