In the early 1800s, a scientist called Charles Wheatstone knew that the slight difference in the image seen by the left eye and the right eye was what gave people depth perception. He wondered, though, what exactly would happen if he showed each eye radically different things?
He imagined that the mind would blend those things together, and so he set up a kind of wedge that would show one eye a face, and the other a house. The people should see a house with a face, he thought. (And if they did, wouldn't it be awesome? Oh well.) Instead, when he put the device up to their eyes, they saw a face, then a house, then a face, and then a house. Instead of blending, the images alternated, and the brain paid attention first to one eye's input and then the other. He called this phenomenon "binocular rivalry."
This basic binocular rivalry set-up has been the basis for a lot of experiments in perception since then. One of those experiments discovered a phenomenon called "continuous flash suppression," possibly because "functionally blinding someone in one eye" didn't have a nice ring to it. The brain doesn't pay attention to everything at once. You, right now, are focusing on this page, and have not noticed the rest of your desk. If your cat were to jump up into your sight, you'd see it, because it moved and caught your attention. If a person were to show one eye a continuous stream of brightly flashing rectangles, or other rapidly changing images, while the other eye is shown a static, or nearly-static image, the static image is suppressed. It's not just a matter of focus — one eye is essentially blinded for a while. And this method has been shown to suppress the static image for longer periods of time than nearly any other technique. Just like the dinosaurs in Jurassic Park, we can sometimes only see what moves.
So if you want to freak someone out, or just have a little fun finding out what it's like to be a cyclops for a while, try suppressing an eye.