"And they went up into the south, and they came to Hebron." (Num. 13:22)

Caleb went there alone and prostrated himself on the graves of the forefathers, that he not be persuaded by his fellow spies to participate in their scheme. (Rashi, based upon Sotah 34b)

When a person in a later generation studies and repeats the words of an early Sage from the Mishna or Gemara, whose life and intellect are those very words, he puts his own life and intellect into theirs. This is called "the joining of spirit to spirit". (Zohar) For through this, he enlivens that Sage and his words. Thus it says: "His lips move in the grave."1

This may be a form of prostration on the graves of Tzaddikim2, for the life-force of the Tzadik is buried and hidden in his words, and the one who studies them, by investing them with his own life and intellect, enlivens the consciousness of that Tzadik. This is called "the joining of spirit to spirit."

[From Me’or Einayim, Likutei Mesechta Shabbat, as translated with commentary from Sefer Baal Shem Tov]