As a child, the 3rd Lubavitcher Rebbe, Rabbi Menachem Mendel, grew up in the home of his grandfather, the Alter Rebbe.[1]
Once, he asked his grandfather the meaning of the verse (Bereishit 26:5) “Because (Ekev) Avraham hearkened to My voice.”
His grandfather responded:
“Through the trial of sacrificing his son, and the rest of the tests, Avraham merited to feel G-dliness in his entire corpus—from his head until his heel (ekev).
Fifty years later—on the 19th of Kisslev 5606—in a chassidic gathering, when Rabbi Menachem Mendel repeated this teaching, he elucidated the idea of self-sacrifice, saying:
“Avraham our patriarch, through his self-sacrifice to spread G-dliness in the world, refined his body to the point that even his heel felt G-d’s command more than the intellectual can “feel” in his mind.”
“While the intellectual can reach the point that his mind “hears,” Avraham reached the point that his heel, felt.”
“My Grandfather”—continued the Rebbe—“in his self-sacrifice to spread the teachings of the Bal Shem Tov, educated the chassidim that they should feel. Meaning that the intellectual understanding of G-dliness should be felt in all the limbs of the body. This is the intent of the verse (Tehillim, 35:10) ‘All my bones shall say, O Lord, who is like You.’ G-dliness should be so clear to the point that one’s bones speak.”
[1] Sefer HaMa-amorim, 5708 Pg. 253-254