Q. Is there a source for the minhag that some people have to point with their little or pinky finger to the Torah when hagbaah is done? Why that finger?

A. Shulchan Aruch (O.H. 134: 2) mentions that at the time of hagbaa the presents should be shown the
writing on the Sefer Torah and they should see it and exclaim: Vezos Hatorah… This is the Torah that Moshe placed before the Children of Israel. Mishna Berura (ibid. 12) rules that the above should only be said in front of the Sefer Torah.
Minhag Yisroel Torah (1: 134: 2) mentions that the expression “ze” and similarly “zos’ are used for pointing with the finger (See Menohos 29a and Chulin 42a).
Yalkut Meam Loez (Ki Savo 27: 26) indicates that it is a minhag to point with the “zeres” or small finger and then to kiss it.
Rav Chaim Falaggi (Sefer Hachaim 3: 6 and Ruach Chaim Y.D. 285. See also Lev Chaim O.H. 167: 6) quotes different Midrashic sources for using the finger and also mentions using the index finger.
It is quoted in the name of Horav Chaim Pinchas Scheinberg zt”l that he gave the following explanation: The Torah lists the ten generations from Noah until Abraham, including Yoktan, who established the largest number of families. Rashi notes that Yoktan merited establishing so many families due to his great humility as his name indicates (from the root katan-little). Horav Scheinberg went on to explain that when pointing at the Torah we take this lesson to heart and we point with our smallest finger – the pinkie – to indicate that we should reach out to try to gain understanding of the Torah with the utmost humility and thus merit to succeed in this aspiration. (Ask The Rabbi – Eish Hatorah, See also Kovetz Beis Vaad L’Chachomim p. 498).

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a