Birkas Kohanim requires heartfelt intention

במדרש אמור להם אמר הקב"ה לכהנים לא מפני שאמרתי לכם שתהיו מברכים את ישראל תהיו מברכים אותם באנגריא ובהלות רק תהיו מברכים בכוונת הלב כו'.

In the Midrash, on "say to them": the Holy One said to the Kohanim, "Not because I told you to bless Israel should you bless them as forced labor (angarya) and in haste; rather you shall bless them with the intention of the heart…"

The Midrash warns the Kohanim not to treat Birkas Kohanim as a mere obligation discharged hurriedly and grudgingly, like a conscripted task, but to perform it with genuine heartfelt kavanah.

פי' שאעפ"י שעיקר כח ברכת כהנים הוא מתנה מן השמים שהקב"ה מברך את בנ"י.

The explanation: even though the essential power of Birkas Kohanim is a gift from Heaven — that the Holy One blesses Bnei Yisrael.

The real source of the blessing is Hashem Himself; the priestly blessing is ultimately a Heavenly gift flowing down to Israel, not generated by the Kohanim.

לא יחשבו הכהנים שאין הברכה מצדם רק לעשות השליחות.

The Kohanim should not think that the blessing is not from their side at all, that they are merely carrying out a shlichus (the role of a messenger).

One might suppose that since Hashem is the true source, the Kohanim are just passive conduits whose own feelings are irrelevant. The Sefas Emes rejects this.

רק באמת רצונו ית' שהכהנים יברכו את ישראל בטוב לבבם.

Rather, in truth it is His will, may He be blessed, that the Kohanim bless Israel with the goodness of their heart.

It is precisely Hashem's ratzon that the Kohanim invest their own heartfelt goodwill into the blessing — their inner participation is part of the Divine design, not incidental to it.

ואז חל על ברכתם ברכת שמים.

And then the blessing of Heaven rests upon their blessing.

The Heavenly blessing attaches itself to the Kohanim's words specifically when those words are spoken with genuine heart — the human kavanah becomes the vessel that draws down the Divine blessing.

ז"ש אמור להם דברים היוצאין מן הלב כנ"ל:

This is the meaning of "say to them" — words that come from the heart, as above.

"Say to them" (emor lahem), with its full spelling, hints at heartfelt speech: the Kohanim must bless with words that issue from the heart, for only such words carry the blessing of Heaven.

Summary: Although the true power of Birkas Kohanim is a gift from Hashem, the Kohanim must not treat themselves as mere messengers blessing by rote; it is Hashem's own ratzon that they bless Israel with heartfelt goodwill, for only words that come "from the heart" become the vessel upon which the blessing of Heaven rests.