One of the most complicated problems in symmetry is to transform something right-handed into its left-handed symmetrical counterpart.
Effecting a transformation from one configuration to another is nearly always possible, but transforming right to left involves completely dislodging the thing from its coordinates and reversing it.
The only way to turn a right-hand glove into a left-hand glove is to turn it inside out.
The transformation from the kelipah to holiness, from deliberate sin to virtue, likewise involves turning everything in one's personal world completely inside out.
Another analogue is "the reversal of the clay under the seal" where there is a reversal from left to right, from engraving to embossing.
This is the "secret of the seal" referred to in Kabbalistic writings, the secret of inversion.
With teshuvah on this level, a person elevates to holiness not only the permitted elements of kelipat nogah but even acts and elements of outright evil.
His transgressions are not merely mitigated but are completely transformed to good.
Thus, also what is forbidden and utterly profane can, in certain circumstances, be rectified even before the "end of days."
How is such repentance achieved?
By rousing oneself to a return to God in a manner coming from the depths of the heart.
The "depths of the heart" is what a person truly desires, as opposed to the things he desires only with the more external aspects of the heart.
A person might say, "I want to study"; "I want to pray"; "I want to perform mitzvot"; but these might not be what he truly desires.
The depths of the heart is what a person would do if nothing were compelling him to act in any specific way, if he were truly free to do whatever he desires.
Teshuvah from the depth of the heart is a teshuvah that reaches to the very core of a person's being, transforming his will all the way down to its deepest essence.
–Rabbi Adin Steinsaltz
From Opening the Tanya by Rabbi Adin Steinsaltz