It can be explained, in the matter of עקב — the “heels” — that within a person there are 3 Parts / ג’ החלקים:

1. Qualities of the mind / מדות המח / Includes all a person’s opinions and views.

2. Qualities of the heart / מדות הלב / includes the passions and desires / תשוקות ורצונות.

3. Qualities of the limbs / מדות האברים// האברים התחתונים, down to the heel / עקב, is the part connected to physical drives / תאוות.
   

The wholeness of a Jew /  שלמות של יהודי is when her Judaism / יהדות  permeates completely through all 3 levels — from the highest intellect to the very lowest physical limb.

As Maran Baal Yesod HaAvodah / מרן בעל יסוד העבודה  said about Rabbi Mordechai Lider זי”ע:
“His Judaism was infused from head to heel” / חדורה מראשו ועד עקבו.

This is the meaning of והיה עקב תשמעון  — a hint / רמז  to the heel / עקב, the end of the body — that hearing / שמיעה  should penetrate all the way to the heel. As Chazal / חז”ל  say: “The ear should be permeated in him up to the heel.”


והיה — is a language of joy / לשון שמחה:

Initials / ראשי תיבות: י’שמחו ה’שמים ו’תגל ה’ארץ — Yism’chu hashamayim v’tagel ha’aretz — whose initials spell י־ק־ו־ק.
This is the greatest joy Above — כאשר עקב תשמעון — when even the heel / עקב  hears.

Not only the mind and heart / מוח ולב  hear the dvar Hashem, but the hearing penetrates into the physical limbs / איברים הגשמיים, down to the heel — the very end of the body.

The Be’er Mayim Chaim / באר מים חיים  explains:
In all physical matters / ענינים גשמיים — eating, drinking, sleeping, walking / אכילה, שתיה, שינה, הליכה — there are the letters י״ה.
In matters of Divine service / עבודת ה’ — service, mitzvah, holiness, Torah / עבודה, מצוה, קדושה, תורה — there are the letters ו״ה.

Seemingly, it should have been the opposite, since י״ה is a higher level /  מדרגה than ו״ה.
But the purpose of Hashem’s will / תכלית רצון ה is that a Jew should infuse the light of Hashem /  שיהודי יחדיר את אור האלקות even into physical matters.

This is even higher than kedusha, Torah or mitzvot, for it reveals Hashem’s light in the lowest places.

Thus, והיה עקב תשמעון — the greatest joy Above is when a Jew is metaken / מתקן all the limbs / איברים down to the heel / עקב, the lowest extremity.

King David feared most the sins of the heel / עוון עקבי — not so much the sins of the mind or heart, but the failure to sanctify the most external parts of oneself.

The path to awaken the limbs

The way to awaken the limbs /  לעורר את האברים and infuse them with the light of Hashem /  אור האלקות is through awe of Heaven / יראת שמים.
While awakening the mind and heart / מוח ולב  can be done through love of Hashem / אהבת ה, the limbs / איברים  can only be awakened through fear of Hashem / יראת ה’.

There are 3 levels of Yira:
Fear of punishment / יראת העונש — knowing there is judgment and a Judge / דין ויש דיין restrains a person from the worst desires.
Fear of sin / יראת החטא — not because of punishment, but because sin itself distances from Hashem.
Awe of exaltedness / יראת הרוממות — and other higher forms of fear.

The rule is: only Yirat Shamayim / Fear of Heaven penetrates the Ohr HaElokut / the light of Hashem into the inner being / פנימיות  of the limbs, all the way to the heel.

Like rain — a sudden heavy rain wets only the surface; only steady rain soaks into the earth.
So too, occasional flashes of inspiration do not last; only consistent awe of Hashem permeates deeply.

והיה עקב תשמעון — Joy

Maran Admor Baal Beit Avraham / מרן אדמו”ר בעל בית אברהם  wrote:
אין והיה אלא לשון שמחה — vehaya always means joy.
Even from the lowest state — שחה לעפר נפשינו ובטננו דבקה לארץ — through joy / שמחה, one can rise and hear Hashem’s word.

By the power of joy / כח השמחה, one hears once, and then again and again — והולך ושומע עוד — until reaching to serve Him with all your heart and all your soul / לעבדו בכל לבבכם ובכל נפשכם.

This is the power of Shabbat / שבת קודש:
Even if the weekdays are dark, Shabbat brings great light / אור גדול.
This is the joy of Shabbat that infuses our limbs with divine light / האור האלוקי.

Shabbat itself is hinted in והיה עק״ב תש״מעון — the letters of Shabbat / שבת  are hidden within.
Through Shabbat, one can fulfill
All my bones shall say:
Hashem, who is like You? / שכל עצמותי
 תאמרנה ה’ מי כמוך.