Answer: The right index finger is lifted when one is in a specific posture of the prayer which is called tashahud. According to authentic traditions, the finger is lifted when one utters the shahadah by saying 'Ashadu an la ilaha illalah'. The meaning of this phrase is 'I bear witness that there is no god but Allah', as such, the lifting of the finger corresponds with the utterance of this statement so as to express one's belief through his/her action that he/she truly believes that there is no god but Allah.

And Allah knows best.
Mufti Waseem Khan