From any of the lighttable modes, you can display a fully zoomed preview of the image that is currently under the mouse pointer by pressing and holding down W. This is useful to more closely inspect an image while rating and selecting images.
Pressing and holding Ctrl+W fully zooms into the image and also identifies any regions of sharpness in the image that may indicate the image is in focus. For this tool to work the input image needs to hold an embedded JPEG thumbnail, which is the case for most raw files.
Regions in the image with a high level of sharpness are indicated with red borders. If there are no such regions found, any regions of moderate sharpness are identified with a blue border. Note that this is not the same as the focus peaking indicator, which is another way to identifying areas of sharpness within an image.
Sometimes pressing W or Ctrl+W may not appear to have any effect – in such cases, click on the image thumbnail and press the corresponding key again.
If you want the full preview to stay in place without having to hold the W key, you can use sticky preview mode by pressing Alt+W (no focus peaking) or Ctrl+Alt+W (with focus peaking). In sticky preview mode, you can zoom and pan within the image in a similar way to the culling mode. Press W again to return to the original view.