The High Resolution Loupe gives a full resolution print preview (actual print size) of anywhere it hovers on the page. The small square is the preview area and the large square is the actual full resolution preview.
Above the spread in Designer View, is the magnifying glass icon.
The Loupe will pop up and start creating the full resolution preview of the spread. This may take a couple of seconds. Once loaded, the preview is instant. Move the preview square around the spread and view a magnified version at full print resolution. This allows verification of sharpness of images at resolution, as well as check alignment and spacing.
See a video demonstration of the High Res Loupe here.