Introduction to this niggun by Sefer Haniggunim:
The famous Chassid, Reb Shmuel Grunem Esterman z”l was a mashpia in Tomchei Temimim Lubavitch. He heard this niggun from an elderly Chossid who was part of a group who sang before the Alter Rebbe.
When he was a young boy, this chossid was blind. His father brought him to the Alter Rebbe in Liozna to ask for a bracha and a cure. As his father made his request, the Alter Rebbe opened a Zohar and told the boy to start reading. At first the boy saw the words but they were blurry. But when the Alter Rebbe urged him, “read loudly!” The boy’s eyes were opened and he began to read normally.
His father left him in Liozna, and since the boy had a great talent for music the Alter Rebbe had him join his choir.
During that time, this chossid heard the niggun K’ayol Ta’arog from the Alter Rebbe, who would sing it at frequent intervals with profound dveikus.
Reb Shmuel Grunem Esterman explained that the beginning of this niggun is the cry of the Neshamah wanting to rise from this world and connect to Divinity (rotzoi). The response is the second part of the niggun: the Mitzvah of Tefillin. This is the return (shuv), drawing Divinity down, connecting one with Hashem in this world. In this vein, the head tefillin draw down the Daled Mochin, as explained in Kabalistic and Chassidic texts.