If you’d like to see both of the before-and-after versions onscreen at the same time, you’ll love this step. To see your photo in a split-screen view, where the left half shows the Before view and the right half shows the After view, just click the Before/After view button at the lower-left corner of the Preview area (circled in Figure 2). Each time you click this button, it toggles through one of the four different before/after views, going from horizontal side-by-side to split screen, and then vertical side-by-side to split screen.
Of course, there are keyboard shortcuts for these views as well. Press Shift-Y to toggle between the horizontal side-by-side before/after view and the horizontal split-screen before/after view.