The practice is not about tapping but either pointing to the table or placing ones hand upon it at that point in the benching. Tapping rhythmically on the table on Shabbat or Yom Tov, could be a prohibited act.
This appears to be an extension of the Minhag Beit HaRav found in the Hagadah shel Pesach of the Lubavitcher Rebbe. In the Hagadah, at the recital of "matza zo" and "maror zeh", the custom is to actually touch the matza and maror.
One might think that it only applies to the Pesach Seder and those food items, but the Lubavitcher Rebbe discusses in many Chassidic discourses the significance of the word "zeh" and relating it to actual pointing with the hand. It is associated with the higher level of "bittul"(self-nullification) that was attained by Moshe Rabbeinu.
Part of the concept of blessings and requests being answered by G-d is that the one requesting is in a state of self-nullification to the Master of the universe. This is the idea of being a "kli rekone", an empty vessel for the blessing. And this is associated with the expression "zeh" as in "Zeh HaShulchan" found in Birkat HaMazone. This placing ones hand upon the table at that point would appear to be an extension of that practice recorded in the Chabad Hagada.
The Hagada is used as a halachic source in many places.