Each video clip in iMovie (whether in a project or an Event) is represented by thumbnail images grouped together in a filmstrip. Each thumbnail in a filmstrip can represent hundreds of video frames (individual images).
To make it easier to find specific frames in a filmstrip, you can step through the filmstrip frame by frame, rather than skimming it.
To step through video frame by frame:
In the Project browser or the Event browser, move your pointer over a filmstrip without clicking.
Press the Left Arrow or Right Arrow key to move backward or forward in the clip in one-frame increments.
If an entire video clip is selected (appears with a yellow frame around it), you can’t step through it using the arrow keys. If a portion of a clip is selected, pressing the arrow keys moves the selection border in one-frame increments.