Screen Flipped Upside down. How?

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I was in the computer lab the other day, and my Friends and i finished our work earlier and decided to leave. We were standing up, to pack our books when my friend dropped his notebook onto the keyboard. When the book had landed on the keyboard, the screen turned dark for a second and flipped. Everything was reversed, even the mouse (when I moved the mouse up, the cursor when down and vise-versa) it was very weird. Does anyone know how to do this? And does anyone know the reason or purpose Windows had added this Command into the System? I find it pretty amusing.