How Round Per Minute can be measured?

In this flash movie there are six disks with different number of black and white slots. All the disks are rotating at the same rate and in the same direction. However you will see some of them rotating faster then the others. You may see one or two not rotating. Even you may see some of them rotating in apposite direction. This is because each disk contains different amount of white and black slots and frequency of your screen refresh may cause of illusion that they are rotating at different angular speed. This principle actually used to measure rpm (revolution per minute) of rotating shafts.