Surrendering one's will to Hashem

במדרש כי יש איברים שהם ברשות האדם ואם האדם זוכה גם הם אינם ברשותו רק ברשות הש"י כמו שהי' ביצחק אף שטעה לברך עשו לא עלתה בידו.

In the Midrash: there are certain limbs that are in a person's own control, and if a person merits, even they are no longer in his own control but in the control of Hashem — as we find with Yitzchak, that even though he erred and intended to bless Eisav, it did not succeed.

Some faculties of a person are normally his to direct, but when one rises high enough, even those are surrendered to Hashem's control. Yitzchak had reached this level, so even his mistaken wish to bless Eisav could not take effect — his very faculties were no longer his own to misdirect.

והוא ע"י מס"נ באמת אליו ית' כמו יצחק בעקידה א"כ רוצה באמת שלא להיות ברשות עצמו ורצון יראיו יעשה וכן בכל אדם כפי קבלת מלכות שמים בכל יום בבקר בלב שלם להיות בטל באמת כל הרצונות לרצונו יוכל לפעול שלא יצא מרשות הש"י כלל ונראה שזה עצמו הטעם שעשה כן הש"י שיהי' הברכה ליעקב ע"י מרמה ולא נתן בדעת יצחק לברכו.

This comes about through genuine mesirus nefesh (self-sacrifice) to Him, like Yitzchak at the Akeidah — for then a person truly wishes not to be in his own control, and "He fulfills the will of those who fear Him." So too with every person, according to the kabbalas ol malchus Shomayim (acceptance of the yoke of Heaven) each morning with a whole heart, truly to nullify all his desires before His will, he is able to bring it about that he never leaves Hashem's control at all. And it appears that this very thing is the reason Hashem arranged that the berachah should come to Yaakov through deception, and did not place it in Yitzchak's mind to bless him.

By offering oneself completely to Hashem — as Yitzchak did at the Akeidah — a person genuinely no longer wants his own autonomy, and then his will and Hashem's will become one. Anyone can reach a measure of this through sincerely accepting the yoke of Heaven each morning and nullifying his desires to Hashem's. This bittul is precisely why Hashem engineered the blessing to reach Yaakov through Yaakov's ruse rather than through Yitzchak's conscious choice.

רק שיהי' הברכה בביטול כל רצונו להשי"ת כי אדם המברך בנו אף צדיק גמור יש לו נגיעה מצד שהוא בנו.

Rather, the blessing should come about through the complete nullification of his own will to Hashem — for a person who blesses his own son, even a complete tzaddik, has a personal bias (negiah) by virtue of the fact that it is his son.

The berachah had to flow purely from Hashem, untouched by Yitzchak's personal will. Even the most righteous father has an unavoidable self-interest when blessing his own child, and that subtle bias would have colored the blessing.

ולכך הי' כאן שלא יהי רצון יצחק לברך את יעקב.

Therefore it was arranged here that it should not be Yitzchak's will to bless Yaakov.

So Hashem removed Yitzchak's intention from the picture, ensuring the blessing carried no trace of a father's partiality.

וכמ"ש בזוה"ק שיתברך מהש"י בלבד כנ"ל:

And as it is written in the Zohar HaKadosh, that he should be blessed by Hashem alone, as above.

The Zohar confirms the point: Yaakov was to receive his blessing directly from Hashem, pure and unmediated by human will.

Summary: When a person sacrifices his own will to Hashem — as Yitzchak did at the Akeidah, and as we do each morning by accepting the yoke of Heaven — even his own faculties are no longer his to misdirect. This is why the blessing reached Yaakov through deception rather than Yitzchak's choice: so it would flow purely from Hashem, free of any father's bias, through total bittul of human will.