There aren't many things I enjoy watching more than a woman experiencing an earth moving orgasm... My limited experience says that a real one starts in her lower abdomen and spreads up through her torso and then into her legs. It's involuntary, it's intense, her skin gets hot/ flushes, and she loses the ability to form words for a while (which is why I have trouble watching women fake their way through a porn scene..). If you're lucky enough to be on the down stroke when it starts, that kitty will put that kung-fu grip on you, and you best hang on for dear life.
Thank you for this thread, y'all. (I tried to find some sisters shaking in *.gif format, but that's a little harder than I thought..)