Today's Tanya is the middle of Perg Zion. Yesterday we learned that the energy of most worldly things are from the level of klippas, the bright klippa, and when a person engages with them for the sake of serving ashem, he is filtering the good from the klippas and elevating it to holiness. However, if he eats or drinks to satisfy the desires of his body and animalistic self, then the energy of that food or drink that is now within him descends to the impure klippas. His body becomes a vehicle for klippa.
But only temporarily, since as soon as he returns to serving ashem, the energy of the food within him is redeemed and elevated to holiness. Because the food he consumed is permissible, mutter, which literally means untied, its energy is not trapped in klippas and can be elevated. Whereas a food or activity, which is forbidden, us or its energy is tied up and trapped in klippas unable to be elevated, until the future time when Hashem will remove all impurity from the world, or until the person experiences a great and intense chivalve out of love for Hashem. This love is a strong feeling of thirst as a result of his distance from Hashem.
In that case, his sins themselves are transformed into merits because it is precisely his distance from Hashem brought about by those sins that awoke this intense love for Hashem more than that of Hashem who did not sin.