In Salah and other acts of worship, which have been commanded to us, there are certainly secrets hidden within them; whether we understand these secrets or not is a different matter. Whether we understand the reasoning for these with the eyes of Shari'ah or not, the people of insight [ahl-e-Nazar] know very well the secrets of every act in Salah. This is because their sight is fixated on the Worshipped [Allah] rather than the worship itself. Along with the procedure of Salah, they also learn the reasoning and secrets behind it. This is why the innermost secrets are revealed to them. They then begin to view the esoteric in the same way they view the exoteric.

The Hadith Sharif informs us that the secret of lifting the index finger in 'at-Tahiyyat' is that at that moment, Shaytan flees because the lifting of the index finger is more severe upon Shaytan than the strike of the sharpest sword.

Imam Ahmed Rida Alayhir Rahmah, in one place, has narrated two Hadith sharif in regards to this

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قال رسول الله صلى الله تعالىٰ عليه و سلم 'الاشارة بالصبع اشد على شيطان من الحديد

[Musnad Imam Ahmed bin Hanbal, volume 2, page 119 - Dār al Fikr Beirut]

RasulAllah salAllahu ta'ala alayhi waSallam has stated: The lifting of the index finger is more severe upon Shaytan than the sharpest words.

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عن النبى صلى الله تعلىٰ عليه و سلم قال 'هى مذعرة للشيطان

[Sunan al-Bayhaqi, volume 2, page 132 - Dār Sadir Beirut]

RasulAllah salAllahu ta'ala alayhi waSallam has stated: It (the lifting of the index finger) is that which strikes fear in Shaytan's heart.

[Fatawa Ridwiyya, volume 6, page 150 - Barkat e Raza Porbandar]

From Imam Ahmed Raza aur Ma'arif e Tasawwuf

Translated by Sayyid Ali Amjadi

http://thesunniway.com/component/k2/item/201-the-secret-behind-the-lifting-of-the-finger