The opacity determines how opaque (see-thru) the selection is. A higher setting is more difficult to see thru, while a lower settins is easier to see thru.

To increase or decrease opacity:

1. Select Selections/Modify/Overall Opacity from the menu bar.
2. Adjust the spin control as desired. Settings range from 1% to 200%, with 1% making the selection almost transparent and 200% doubling the normal opacity of 100%.
3. Click ok. (you won't notice any change to your image).
4. Hit the delete button on your keyboard once.
5. Copy/Paste into an new window.

The three images above were done using the copy/paste method. The first image is the original. The other two demonstrate an opacity setting of 30%. The middle image had the affect applied to the entire image while the one on the far right had it applied only to the selection, (the box with the middle cut out). The 'test' text was added to a blank window prior to pasting the opaque image to demonstrate the 'see-thru' effect.