To demonstrate the principle of Automatic Feedback.
Place the ends of a yardstick on your two index fingers as shown in the diagram. Then approach the fingers slowly together, trying your best to make them meet at some spot other than the centre of the stick. This will be found impossible. When one finger gets slightly ahead of the other the weight of the stick on that finger increases which then increases the amount of friction between finger and stick and causes the stick to stop sliding until the other finger catches up. By slowing up a finger that starts to get ahead, it keeps the fingers moving toward its centre at approximately the same speed and thus the stick serves as a feedback device.