This achieved emphasis by creating a focal point using contrast. By making the background so colorful the model in black and white stands out but is still unified by pulling the color into the sunglasses.
I haven't checked the actual web-site as I don't use Photoshop, but I love the effect and can see me using this myself. Where I pinned it from said "How to Create a "Double Exposure" in Photoshop — Daily Epiphanie"
Abstract Watercolor Wallpaper in Photoshop. My PaintShop Pro doesn't have a Watercolor effect, so I used colored pencil and played around with the mask, etc. Love taking these tutorials and doing my own thing with them in PSP.