The Conservation Of Angular Momentum

If you have Java, click on the swinging ball to change the string length.

Take a ball attached to the end of a string and move it in a steady motion back and forth, like a pendulum. Once a steady swing has been established, take the thumb and forefinger of your other hand and make a circle around the top of the string. This will not interfere with the swing, so no change will be noticed.

While keeping your swing steady, slowly begin to raise the hand holding the string. As your hand goes higher, the string begins to hit the side of the circle made by your fingers. As this happens, the frequency of the ball's movement increases. The shorter the string, the faster the ball travels and the further from the vertical it will go. As you lower the string back down, the ball will eventually return to its original motion.

The reason for this is that the energy in the system remains constant as the length of the string grows shorter. This produces a longer and faster arc.