When you play a clip in the browser or a movie in the timeline, your video plays in the viewer.
In addition to playing video, you can skim video by moving the pointer back and forth over clips. Skimming is useful for quickly previewing video or finding a particular moment.
In the iMovie app 
on your Mac, do one of the following:
Position the pointer over a clip in the browser or timeline where you want to begin playing, and press the Space bar.
Click a clip in the browser or timeline where you want to begin playing, and then click the Play button 
below the viewer.
To stop playback, press the Space bar, or click the Pause button 
below the viewer.
In the iMovie app on your Mac, click in a movie in the timeline or a clip in the browser to position the playhead where you want to begin playback.
Click the Play Full Screen button 
below the viewer.
To leave full-screen view, press the Esc (Escape) key, or click the Exit Full Screen button 
below the viewer.
In the iMovie app on your Mac, select a clip or part of a clip in the browser or timeline, and choose View > Play Selection.
In the iMovie app on your Mac, choose View > Loop Playback, so that there’s a checkmark next to the menu item.
Position the pointer where you want to begin playback, and press the Space bar.
If the pointer is over a clip in the browser, the clip plays in a continuous loop. If the pointer is over a movie in the timeline, the entire movie plays in a continuous loop.
Tip: To loop a portion of a clip or movie, select part of a clip in the browser or the timeline and choose View > Play Selection.
In the iMovie app on your Mac, choose View > Audio Skimming.