Teshuvah returns sin to its root

בזוה"ק פ' אמור פי' כסוי חטאה שהחטאים נשלכים במצולות ים דשם שורש כל החטאים ע"ש.

In the Zohar HaKadosh, Parashas Emor, it explains "the covering of sin" (Tehillim 32:1) - that the sins are cast into the depths of the sea, for there is the root of all sins; see there.

The Zohar reads "kisui chata'ah" (the covering over of sin) as the sins being hurled into the ocean depths - the place that is the source-root of all sin. To "cover" the sin is to return it to its origin, removing it from the person.

וזהו ענין השעיר המשתלח שמקבל כל עונות בנ"י.

And this is the matter of the sa'ir hamishtaleach (the scapegoat sent away) which receives all the sins of Bnei Yisrael.

The Yom Kippur goat sent to the wilderness embodies this very idea - it takes upon itself all the avonos (sins) of Bnei Yisrael and carries them back to their place of origin, away from kedushah.

דע"י החטאים מתערב הסט"א ובא לקרב אל הקודש.

For through sins the Sitra Achra (the "other side," the force of impurity) becomes intermingled and comes to draw near to the holy.

Every sin gives the Sitra Achra a foothold, allowing it to mix in and approach the realm of kedushah. Sin is dangerous precisely because it grants impurity access to what is holy.

וזהו תכלית כל האיסורים שבתורה שלא ליתן מקום לסט"א להתקרב שעי"ז מסתלק הקדושה.

And this is the purpose of all the prohibitions in the Torah: not to give the Sitra Achra a place to draw near, for through that the kedushah departs.

Every issur (prohibition) in the Torah serves one aim - to deny the Sitra Achra any opening. When impurity gains entry, holiness withdraws; the mitzvos of restraint guard that boundary.

ולכן ע"י תשובה מחזירין כל הפסולת לסט"א וזהו רמז ישיב הרע לשוררי.

Therefore, through teshuvah one returns all the refuse to the Sitra Achra, and this is hinted in "He will return the evil to those who lie in wait for me" (Tehillim 54:7).

Teshuvah works by sending the "waste" - the impurity that attached itself through sin - back to its source in the Sitra Achra. The pasuk hints that the evil is returned to the very forces from which it came.

וכ"כ ישוב עמלו בראשו וכשחוזר לשורשו בנוקבא דתהו"ר נק' כסוי חטאה.

And so too, "His mischief shall return upon his own head" (Tehillim 7:17) - and when it returns to its root, into the chasm of the great deep, this is called "the covering of sin."

When the evil is sent back down to its root in the abyss, the sin is "covered over" - sealed away in its source and no longer clinging to the person.

וממילא חוזר האדם להתדבק בקדושה:

And automatically the person returns to cleave to holiness.

Once the impurity has been returned to its origin, nothing stands between the person and Hashem - and he naturally regains his deveikus (cleaving) to kedushah.

Summary: Drawing on the Zohar, the Sefas Emes explains the avodah of Yom Kippur and the sa'ir hamishtaleach: sin gives the Sitra Achra an opening to encroach on holiness, and the Torah's prohibitions exist to block that. Teshuvah reverses the process, casting the impurity back to its root in the depths - "covering" the sin - so that the person is once again free to cleave to kedushah.