How does the Overlay/High Pass technique work?

What will be an ideal response?

When you duplicate an image and overlay it over itself, it dramatically increases contrast overall and especially at the edges. The Overlay/High Pass technique essentially does just that-overlays an image on top of itself-but only at the edges. The filter works by filling the layer with 50% gray, which will become transparent when overlayed. The only areas that aren't filled with gray are edge areas. The more you increase the Radius value, the greater the area of the image that Photoshop will define as an edge area. When you apply the Overlay mode to the filtered artwork, most of the filtered artwork becomes transparent, because it's been filled with gray. The edge areas-which aren't filled with gray-remain visible to be overlayed, thus increasing the contrast dramatically.