Why would you flip a movie? Flipping lets you subtly change the impact of the visual image. If the movie shows motion, having it move from left to right makes it seem natural (since we read from left to right). Flipping the motion so it moves right to left will make the motion seem slower. Similarly, having a spokesperson on the right side of the screen is a stronger visual position than having him on the left. You can flip just sections of a movie, too, which gives you greater control over the final product.
Open the project you want to work with in iMovie or create a new project. You can only flip movies when they are in projects--you cannot flip the original clip.
Drag the clip into the project.
Move the cursor over the clip, and a menu with a gear appears on it.
Click on the "Gear" menu and a pull-down menu appears. Choose "Clip Adjustments" from this menu.
An "Inspector" window appears. Click on the big button labeled "None" next to "Video Effect."
Click on "Flip," which is the second from the left on the top row. You can move your cursor over the clip to move back and forth through the clip with the effect applied, so you can preview what it will look like.
The "Inspector" window appears again, only now "Flipped" appears next to "Video Effect." Click on the "Done" button in the lower right corner to finish.