What is the reason for moving the index finger during the Tashahhud, and what is the correct way to do so?

There was a previous discussion about how to point with the forefinger. As for the wisdom behind moving it, it has been said that it is a repellant for Satan. This has been narrated as being attributed to the Prophet, peace and blessings be upon him, with a chain of transmission that is not sound, and it has also been narrated from Mujahid. It has also been said that it is a sign of monotheism, and this has been narrated from Ibn Sirin and from Ibrahim. It is not unlikely that it serves both purposes.