Moving the index finger is something that is neglected by many worshippers because they do not know the benefits or its effects on khushu.
The Prophet, peace and blessings of Allah be upon him, said:
“It is more powerful against Shaytan than iron.” (Ahmad 2/119)
Because of its great benefits, the Sahabah, may Allah be pleased with them, used to enjoin one another to do this and were very keen to remember to do this act that many people nowadays take very lightly. It was reported by Ibn Abi Shaybah that the Companions of the Prophet, peace be upon him, used to enjoin one another with regards to pointing with the finger during du’a.
The Sunnah in pointing with the forefinger is that it should remain raised and moving, pointing towards the qiblah, throughout the Tashahhud. And Allah Knows Best.