Mitzvos That Guard Blessing
במדרש אם שמור תשמרון כו' כל המצוה ק"ש שבת כו'
In the Midrash it expounds the verse "If you will surely keep [all this mitzvah]" (Devarim 11:22) — "all the mitzvah" refers to Kerias Shema and Shabbos, and so forth.
The Midrash on the verse about keeping "all the mitzvah" identifies it with Kerias Shema and Shabbos, mitzvos that somehow stand in for the whole.
בשם אמו"ז ז"ל ענין אמרם שקולה שבת ציצית ככל המצות
In the name of my mother's father, of blessed memory, regarding what Chazal said, that Shabbos and tzitzis are each weighed against all the mitzvos.
The Sfas Emes cites his grandfather on the teaching that Shabbos and tzitzis are each considered equal in weight to all the mitzvos combined.
הפי' שכל מצוה יש לה ענין מיוחד
The explanation is that every single mitzvah has its own particular dimension and function.
He begins by establishing that ordinarily each individual mitzvah has its own specific role and area of influence.
רמ"ח נגד רמ"ח אברים
The two hundred and forty-eight positive mitzvos correspond to the two hundred and forty-eight limbs of a person.
He maps the 248 positive mitzvos onto the 248 limbs of the body, so that each mitzvah corresponds to and affects one limb.
אבל אלה המצות הם מביאין הארה וקדושה לכל הרמ"ח אברים ע"כ
But these mitzvos bring an illumination and a kedushah to all the two hundred and forty-eight limbs together — and therefore they are weighed against all the mitzvos.
Certain special mitzvos like Shabbos and tzitzis are different: they radiate kedushah into all 248 limbs at once, which is why they are weighed against all the mitzvos.
וז"ש שמור תשמרון שע"י מצות אלו נשמר כל האדם
And this is the meaning of "If you will surely keep" — that through these mitzvos the entire person is guarded and kept whole.
"You shall surely keep" thus means that through these all-encompassing mitzvos the whole person is protected and made whole.
ולפרש סמיכות המד' לכאן
Now to explain the juxtaposition of this Midrash here, to this place in the parsha.
He now turns to explain why this Midrash is connected to this particular spot in the parsha.
דכתיב הברכה אשר תשמעו כי השי"ת כן רוצה לברך את בנ"י תמיד אבל כל ברכה צריך שמירה כמ"ש יברכך ה' וישמרך
For it is written, "the blessing, that you shall listen" (Devarim 11:27) — for Hashem indeed wishes to bless Bnei Yisrael always; but every blessing requires guarding, as it is written, "Hashem shall bless you and guard you" (Bamidbar 6:24).
The parsha speaks of the blessing tied to listening; Hashem always wants to bless Bnei Yisrael, but every blessing needs guarding, as in Birkas Kohanim where blessing is paired with being guarded.
שצריך להיות שלא יהי' ח"ו נגרר שום רע ע"י הברכה
For there must be no evil drawn along, chas v'shalom, by means of the blessing.
The guarding is needed so that no harm should come riding along with the blessing itself.
כמ"ש ושבעת השמרו לכם
As it is written, "and you shall be satisfied — guard yourselves" (Devarim 11:15-16).
He brings the verse warning that after satisfaction one must guard oneself, showing that abundance carries a danger requiring protection.
ולכן ע"י שמירת המצות
And therefore it is precisely through the guarding of the mitzvos,
Therefore it is specifically through keeping the mitzvos,
האדם נשמר
that a person is himself guarded and kept whole,
that the person himself becomes guarded and whole,
ובכחו לקבל הברכות
and he has within him the power to receive the blessings.
and only then is he a fit vessel able to receive the blessings.
ולכך הברכה הוא אשר תשמעו:
And it is for this reason that the blessing is "that you shall listen."
This is why the Torah frames the blessing as "that you shall listen" — listening and keeping the mitzvos is what makes one able to hold the blessing.
Summary: The Sfas Emes, citing his grandfather, explains why Shabbos and tzitzis are each weighed against all the mitzvos. While every mitzvah ordinarily corresponds to one of the 248 limbs and affects it alone, these special mitzvos pour an illumination and kedushah into the entire person at once, which is why "keeping all the mitzvah" guards the whole person. He then ties this to the parsha's theme of blessing: Hashem always wishes to bless Bnei Yisrael, but every blessing requires guarding so that no evil is drawn along with it, as we see in Birkas Kohanim and in the warning to guard oneself after satisfaction. Therefore it is precisely through the guarding of the mitzvos that a person himself is guarded and made into a fit vessel — and this is why the blessing is bound to "that you shall listen."