When you select this button, the cursor will change into a magnifying glass with a circle around it. When you pick a point on image, the area under the circle will show your photograph as it was when you opened it as shown below.
Click again to move the mouse to a different area to show.
While in this mode, you can zoom in and out using the middle mouse wheel, and pan the image using the scroll bars.
Select the button again to turn off this mode.