In addition, the underside of the water outside of the window appears as a mirror, reflecting the bottom. In stillwater environments, this mirror may be very smooth. In addition, in glassy, stillwaters, the window has well-defined edges. This means that in stillwater areas a fish will have optimal vision, allowing it to see food (and you) as best as possible. In broken water, however, the view is anything but serene, with the window becoming degraded. This can work to your advantage as an angler because it can make your image blend in better with the window.
An object depressing the mirrored surface outside of the window changes the angle of the surface film, allowing light to pass through. If fishing floating flies, you have a better chance of the fish noticing and selecting your fly pattern if it depresses the surface in the same manner as the natural.
When light strikes the water's surface some of it will reflect off. Light reflecting off the water's surface creates what we call glare. On blue-sky days, polarizing glasses can help to eliminate much of this glare (don't fish without them). On cloudy days, however, light becomes scattered, creating "white glare." Polarized glasses will help, but can't completely eliminate such fish-concealing shine.
As touched on earlier, light entering the water at increasingly lower angles (relative to the water's surface) bends at increasingly steeper angles, creating a distorted effect from within the window. Crouching down 30 feet from the edge of the fish's window will make you appear as little more than a squat toad on the window's edge. Of course, wearing highly-reflective clothing (a bright white shirt, for example) can still make you a rather noticeable squat toad.
So at low viewing angles (to the fish), you are squat, perhaps out-of-focus, and simply less visible. So why do fish some still spook when you cast at them, even if you're 30 or 50 or 70 feet away? Because you cast at them. Any sudden motions may be noticed, especially the rod waving about and the flash of the line as it rockets through the air and detonates on the water mere inches from the quarry. If you wish to error on the side of caution when approaching a fish, assume that the fish will have the ability to see you or some part of you (or your equipment) and act accordingly. That means moving smoothly, wearing clothing that blends with the environment, casting in a more side-arm manner (if feasible), using lines that aren't glowingly reflective, reducing false casting, and not laying the line down directly on top of the fish (try a Reach Mend, for example). There may be times when you can use cover to great advantage and times when the fish are not overly bothered by your presence, but I would rather be conscientious when approaching a big fish (especially in smooth water) than risk spooking the fish because I rushed in.
Snell's Window is a truly invaluable piece of presentation knowledge. Learn to use it to your advantage and you may soon enjoy greater fly fishing successes.