When you expand time in a video, you are making the duration of the video sequence longer than real-time. This is relatively rare — it's not often that you want your audience to see something more slowly than it actually happened.
However there are some obvious exceptions. Perhaps the most common is the sports slow-motion replay, where an action sequence that took place in real-time over a few seconds is slowed down and repeated until it takes half a minute or more. This type of time-stretching is used to add impact, additional information, or to help the viewer process information that would be too fast in real-time.
In extreme examples such as the assassination of President Kennedy, entire documentaries have been based around a few pieces of footage lasting only a few seconds each.
There are several common tools for expanding time...
Simply slowing down the speed of a video clip increases the duration.
This technique became popular around the beginning of the 21st Century as mid-level editing software packages began including the capability. Basically it means that the editor is not limited to setting a single speed for a video clip; rather, the speed can be varied smoothly over time. A scene can start off in real time and gradually (or quickly) increase in speed. The speed can go up and down as the editor pleases.
An early example of time remapping was the movie The Matrix. Fight scenes in this movie used a lot of time-remapped slow motion to add impact.
For an example of how time remapping works see time remapping in Adobe Premiere.
Repetition, Different Angles
The same shot can be shown from multiple angles and repeated. This is common in sports and is also used in genres such as action films (watch how explosions are shown from different angles).
Alternatively, things that happen simultaneously in different locations can be shown in sequence. In Pulp Fiction, for example, a number of scenes are shown one after the other that are supposed to have taken place either at the same time or in a different order.
Adding cutaways and different angles can drag out an event that would be very quick in real time. An example in reality-TV programs is the "announcement of the winner". After a very long build-up to the final moment where the winner (or loser) is announced, the editor wants to milk the climax for all it's worth. There is typically a long wait between saying "...the winner is..." and the actual announcement. If only one wide shot was shown during this pause it would look unnatural but by using a series of shots (contestants looking anxious, judges looking pensive) the audience accepts the wait and engages in the suspense.
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