Lightroom Integrated: Workflow Phase II - Black and White in Photoshop CS4 and Photoshop Lightroom - page 83

Compare View
You can use the arrow keys to scroll through the candidate images, the
swap icon to exchange the candidate image with the select image, and
the select icon to designate the candidate image as the select image.
When you are i nished comparing images, click the Done button to exit the
Compare View.