To explain things a bit further (with a picture!)...
Look at the photo below:
This was shot at 1/40th of a second - well within the bounds of motion picture cameras.
Now, think about the two obvious points of motion that are going on.
1 - The bat is moving left to right across the frame in an arc.
2 - The ball is moving downward diagonally.
In normal motion pictures, the next frame would have very similar blurring with the start of the blur beginning right where the old blur left off.
With improper tweening, there would merely be a new frame inserted between the two which would contain half the blur from the first frame and half the blur from the second frame... GREAT! Except, for every frame seen on screen the motion would appear jerky because the ball and bat would actually move halfway BACKWARDS through motion that it has already covered. While not something someone may easily identify, it will definitely be noticable, and may be part of the complaints about the new 120hz displays that perform some sort of tweening process.
The proper way to create a tween frame would be to take the photo above and actually de-blur it by 50%. This would require restoration of the background and proper determination of the motion of the different objects in the frame. That is, the ball going one direction would have to be shown over a period of 1/80th of a second (or less) and the bat would have to be shown, as well, over a period of 1/80th of a second (or less). Then the new frame would need to occur duing the next 1/80th of a second and contain the blurring that was removed from the first frame - along with, perhaps, some white space to account for the shutter closing during the film.
Full, and proper implemenation of 120hz frame tweening from a 24fps (hz) source, would actually require about a 5 fold reduction in blurring and then 4 new frames be created with the proper blur on each and every frame.
...
or - just repeat the same frame five times and call it a day with proper 24hz playback.