Thus, what we have learned [from the previous section is as follows]:
The main part of the gilgul associated with the body is specifically that part which was damaged. The other parts of the soul previously rectified in other reincarnations only return as aspects of ibur. When the part which is associated with the body performs a mitzvah in this world, the other parts share a portion of the mitzvah, since they have assisted in the performance of the mitzvah, as explained earlier concerning the soul of a righteous tzadik that comes as an ibur. Since they only come to assist for the sake of good they do not share the punishment when the main Nefesh transgresses.
When a person reincarnates, the entire Nefesh returns. However, the essence of the gilgul is only the part that sinned in a previous body and returns to become rectified. It is associated with the body, and reward and punishment are applicable to it. However, the rest of the Nefesh takes part in the reward but not in the punishment.
Now, the Nefesh in its entirety suffers and receives punishments in the existing body, besides what was suffered by the sparks in previous bodies. And it will suffer as well the pain of death and the pain of what comes after death. Consequently, there is atonement for the earlier ["light"] sins. Moreover, through the mitzvot of previous gilgulim and the present one in which they have a portion, as we have said, the tikun of the Nefesh becomes complete.
In this respect, the comparison between the mostly rectified parts of the soul that come as an "ibur from birth" and the soul of a righteous tzadik that comes as a normal ibur ceases. For a soul of a righteous tzadik, the ibur is not meant to act as a cleansing process. It just increases its own merit by the ibur, and it assists the person in which the ibur is occurring. However, by the case of "ibur from birth", the mostly rectified parts of the soul need the ibur to complete their atonement and tikun.
However, if they had a portion in the [present] sins as well, there could never be tikun for the Nefesh. A person usually sins, and this would only add sin to sin and there would be no end to it!
If the mostly rectified sparks had a portion in the sins of the non-rectified sparks, then it would hamper progression towards tikun, because previous tikunim would be cancelled out by future sins of other sparks.
…through reincarnation, completion ...for all the sparks from the "head" of the nefesh to its "feet."
However, since the rest of the Nefesh does not share responsibility in the evil of this spark -- only in its merits, sins can be atoned for and not increased. New merits can be added through each gilgul. There can be a conclusion to the cycle of reincarnations and tikun for theNefesh. Understand this well.
In this way, the Nefesh achieves completion in all its sparks; through reincarnation completion is achieved for all the sparks from the "head" of the Nefesh to its "feet".
This refers to the beginning of the discourse in Section 2. The sparks of the Nefesh from the head to the feet are the sparks of the soul-body of Adam HaRishon. Each generation is destined to rectify the selection of sparks associated with it, until the tikun of the whole is completed.
Once the "feet" are reached, then Mashiach will come, as it says in the Zohar (Parashat Pekudei pg.258, and the end of Parashat Vayakhel).
In the age of Mashiach, there will be no more need for gilgulim. Mankind will be fully rectified and will have merited the appearance of the greatest soul of all, the soul of Mashiach, the keter of human souls.
[Commentary by Shabtai Teicher.]