How transitions are created
Transitions require overlapping video from the clips on each side of the edit point. A one-second transition requires one second of video from the end of the left clip and one second of video from the start of the right clip. When you apply a transition to an edit point, Final Cut Pro attempts to use additional video from each of the clips to create the overlap. These additional (unused) sections of clips are known as media handles. The following image shows a clip with media handles on each end and a clip with no extra video, or media handle, at its start.
The media handle video may not be appropriate for use in a transition. For example, if the video includes unusable video such as a slate, it will be visible in the project.
You can set the default duration for transitions. See Set transition defaults.
The illustration below shows how transitions are created when the clips on either side of the edit point have media handles. The transition is placed so that it spans the clips; one half of the transition overlaps each clip. Unused media from the end of each clip is added to fill out the transition. The total duration of your project is unchanged.
If one or both of the clips do not have enough extra content (media handles) to support a transition of the default duration, you are given the option to overlap media in your project to create the transition.
Final Cut Pro uses any available media handles and then overlaps media as necessary to create the transition. The illustration below shows a transition created without media handles. The two clips are made to overlap, and the transition is placed over the overlapping region. This is effectively a ripple trim edit, and the total duration of your project decreases.