iMovie '11: Trim unwanted frames from a video clip
Some of your video clips might have frames that you don’t want to include in your project. You can easily remove these frames by trimming the clips.
To trim a video clip:
In the Project browser, select the frames of a video clip that you want to keep.
Choose Clip > “Trim to Selection”; the unwanted frames are removed from the project.
The Clip menu appears in a light gray bar across the top of your computer screen.
The deleted frames are removed from your project only, not from the source video in your Event Library. Therefore, at any time, you can restore any of the trimmed frames to the clip in your project. You can press Command-Z immediately after you trim the frames to restore them all. Or, you can find the source video in the Event Library and add the trimmed frames back to your project.