For doing tilak, I have been always instructed to use my ring fingers. But my Panditji always used his thumb. When I asked about it he instructed me that Pandits and brahmins can also use their thumb too, but I didn't get the reason for it.
So I really want to know the logic behind it.
From an article by bhaskar.com (emphasis mine)
The area below the ring finger is called the 'surya parvat' or 'surya shetra' and the 'agaya chakra' is situated in the forehead. It is believed that by applying tilak with the ring finger gives 'surya' or sun like glow to the face. This also awakes the 'agaya chakra' and the person gets wisdom and fame.
This is why tilak should always be applied with the ring finger.
It is believed that by using the ring finger, a person brings a glow to the face – symbolically the energy of Surya (the Sun). This makes the person ready for the particular day’s Karma (act, object).
Pundits also use their thumbs to apply tilak (commonly on other people).
... at the base of the thumb is Venus. Saturn is the protector of lifespan, Jupiter represents immortality and Venus represents health. Tilak applied with the ring-finger and the thumb is very beneficial. Venus gives life force, health, is the creator of new life, the bestowed of gentility and politeness. This is achieved by applying Tilak with the thumb. unbroken rice grains (Akshat) mixed in red chandan and apply the Tilak with the thumb.
Source: Tilak - why is it applied in spiritual practices? on SpeakingTree.in
Devi Bhagavatam mentions some results in Book 11 for applying Urdhva Pundra (Tilak worn by Vishnu Devotees) with different fingers.
According to the chapter,
Thumb : Nourishment is obtained
Middle Finger : Longetivity is increased
Ring Finger : Food is obtained
Forefinger : Liberation is obtained
Below is an excerpt of Chapter 15:
If the Ûrdhapundra be drawn by the thumb, nourishment is obtained; if it be drawn by the middle finger, longevity is increased; if it be drawn by nameless or ring finger, food is obtained and if it be drawn by the fore finger, liberation is attained. So the Ûrdhapundras ought to be drawn by these fingers, only be careful to see that the nails do not touch at the time of making the mark.
In the beginning of the chapter, Lord Vishnu also says how the sacred ashes (Bhasma) are to be applied. While applying fore, middle and ring fingers of the right hand fingers are used.
One is to hold the Tripundra, six Angulas high or greater by the fore, middle and ring fingers of the right hand. If anybody uses Tripundra, shining and brilliant, and extending from eye to eye, he becomes, no doubt, a Rudra. The ring-finger is the letter “A,” the middle finger is “U” and the forefinger is “M”;
But I have observed that elders and some priests mostly instruct us to apply tilak with three fingers viz Ring finger, Middle finger and thumb and not the index finger and little finger.
