Shaykh Ibn 'Uthaymeen writes in his book ash-Sharh al-Mumti':
The scholars have said that you point with your index finger when there is a reason to do so. But this begs the question, what classifies as a reason?
We say, its reason is when Allaah is mentioned, but the scholars differ on when this actually is. Some say this means whenever the name of Allaah is mentioned in the Tashahud (i.e. you raise your finger everytime the name of Allaah is mentioned).
Whilst others say, it is only when the kalimah laa ilaaha illallaah is said that one raises his finger.
This is the difference between the scholars, however the Sunnah teaches us that one should raise his finger only during the time of du'aa, since the wording of the hadeeth is:
He moves it (the finger) and supplicates with it.
[Imaam Ahmad, Nasaa'ee, al-Bayhaqee and Ibn Khuzaymah]
However both moving the finger as well as not moving the finger have been reported in different hadeeths. The answer to this is easy, (and it is) that the hadeeths mentioning not moving the finger, this means moving it all the time non stop. And as for the hadeeths that mention moving the finger then this refers to moving it during the time of du'aa only.
So whenever you make du'aa move your finger, and this is closer to the Sunnah.
[ash-Sharh al-Mumti' 'ala Zaad al-Mustaqni']