As you move the pointer over a video clip to skim it, the iMovie viewer displays the frame that’s currently under the pointer. You can pause skimming so that the viewer continues to show the same frame of video even as you move the pointer around.
To pause video skimming:
In the Project browser or Event browser, position the pointer, without clicking, over the frame in a video clip you want to show in the iMovie viewer, and then hold down the Control key.
You can now move your pointer anywhere without changing the image showing in the viewer.
After you release the Control key, you can continue skimming video as usual.