Typically when you stretch a video in time, the result looks jerky; you see pause, then movement, then pause again, and so on. This is because a simple time-stretcher operates on original video frames.
What a slowmo effect does is generating additional frames between the original frames, and these new frames add the "missing" movement. So the result is a smooth timestreched video.
Mathieu Duponchelle created a plug-in for GStreamer that implements slowmo video effect. His work is based on the variable videorate code by Joris Valette, a GSoC 2013 student.
In this video the left part is basic time stretching that looks very clunky. The right part is where true slowmo effect is used, it's a lot smoother.
The patch is here: bugzilla.gnome.org/show_bug.cgi?id=708545
An effect for the Pitivi video editor is to be created.
Original videos by Mathieu:
youtube.com/watch?v=jr7EqrLkc6o (simple stretching)
youtube.com/watch?v=6OQbMjqLa8A (true slowmo)