As a general rule, Nefesh, Ruach and Neshama cannot be totally rectified in a single lifetime. Instead, the person must first die. Then the Nefesh is born into a new gilgul in which he will acquire the level of Ruach. Similarly, after the Ruach is also rectified, the Nefesh and Ruach cannot receive the level of Neshama that pertains to them until they come into a new gilgul in which the person will merit the acquisition of Neshama. The Nefesh of a tzadik…will descend into this person's body…

However, when the Nefesh alone has been rectified (without the possibility of Ruach joining with it) if the person is sufficiently worthy by virtue of his efforts in divine service, it may happen that the Nefesh of a tzadik who has perfected himself and does not need to come into gilgul will descend into this person's body, eliminating the need for him to first die. The Nefesh of the tzadik then fulfills the role of the person's own Ruach, allowing the person to transcend the limitations of his own soul root. (See Tanya, end ch. 14) It can even happen that the souls of much earlier generations, all the way back to our father Abraham, will enter the soul of a person, guiding him to greater heights in his spiritual endeavors. (Sha'ar HaGilgulim, hakdama 2)This all depends primarily on the person's performance of mitzvot, since the mitzvot have the power to elicit the Nefesh of a tzadik to whom that mitzvah was particularly important or beloved.

Furthermore, even the Ruach of a Tzadik can be drawn down through the proper performance of a mitzvah. Similarly, it can also happen that the Nefesh of another greater tzadik also comes into the same person, so that the person now has his own Nefesh, the Nefesh of the first tzadik that fulfills the role of the person's own Ruach, and the Nefesh of a second tzadik that fulfills the role of the person's Neshama.

This phenomenon is called "ibur" - literally, "impregnation" - i.e., spiritual possession of a lower soul by a higher soul (or souls) to the benefit of both of them. The benefit to the carrier of the ibur is that his soul becomes greatly purified and refined by the presence of the tzadik's soul, which aids him to reach far higher levels in the performance of mitzvot and attainment of holiness. Thus when he eventually dies, his soul rises to the level of the tzadik's soul in Gan Eden.

There is also a benefit to the tzadik whose soul enters the person living in this world. Since the tzadik helps the person reach greater heights in mitzvot and achieve a greater intensity of holiness, the tzadik's soul also acquires a part therein. This is the secret of the statement of our Sages that "tzadikim are great, for even after their passing they merit children…", (Chullin 7b; Sanhedrin 47a) for they become like fathers to their hosts. (Sha'ar Hagilgulim, hakdamah 2) If he subsequently sins, the tzadik's soul will leave…

Of course the continued presence of the tzadik's ibur depends upon the person's deeds. If he subsequently sins, the tzadik's soul will leave. However, due to the overwhelming holiness of the tzadik's soul within him, guiding him to ever greater heights, it is extremely unlikely that a person will in fact transgress any Biblical or Rabbinic mitzvot.

The following passages from the Zohar discuss the ibur of the souls of Nadab and Abihu, two of Aaron the High Priest's sons, who died suddenly, consumed by a holy flame, when they made an incense offering that was not called for. Although on one level this was regarded as a sin, nevertheless the Zohar interprets it in a very positive light as well: their deaths atone for the Jewish People throughout the entire exile. It is for this reason, states the Zohar, that we read about the offerings and deaths of Nadab and Abihu (Lev. 10:1-2; 16:1ff.) on Yom Kippur, the Day of Atonement, the holiest day of the year. (Zohar, vol. II, p. 57b)

The verse states: "Elazar son of Aaron took for himself from the daughters of Putiel as a wife, and she bore Pinchas; these are the heads of Levite clans, according to their families". (Ex. 6:25) The Zohar asks: Why does the verse state these are the heads of the clans in the plural? He, Pinchas, was only one person, and so it should have said, "This is the head of the Levite clans…." - in the singular. The Zohar answers: That which the heads of the Levites had lost was found in him. The deficiency that they [Nadab and Abihu] caused, and for which they were burnt up by the flame that shot out of Heaven, he [Pinchas] completed. It was for this reason that he earned their Priesthood. Only Aaron and his sons had been anointed as priests. Only their offspring that would be born after this time would automatically become priests. Although Pinchas was the son of Elazar, one of Aaron's sons, he was already alive at the time that Aaron and his sons were appointed to the Priesthood, and he was therefore automatically excluded. However, because Pinchas zealously took up G‑d's cause among the Israelites and assuaged His anger with them for their sinning with Moabite and Midianite women, Pinchas was bestowed with Priesthood. (Numbers 25:1-15) Accordingly, the verse states, "these are the heads" in the plural, not in the singular. (Zohar II, p. 26b) Pinchas inherited not only the Priestly status of Nadab and Abihu, but their very souls…

From the following passage in the Zohar, we gain deeper insight into the Zohar's explanation of the above passage: Pinchas inherited not only the priestly status of Nadab and Abihu, but their very souls, which came into ibur within him:

Rebbe Shimon bar Yochai was sitting and studying parashat Pinchas. (Numbers 25:1-15) Rabbi Elazar his son came before him and asked: What did Nadab and Abihu have to do with Pinchas? Had Pinchas not been alive when they passed away, and he would have been born afterwards, then one can say that they would be reincarnated in him. Pinchas would simply be their gilgul and in this way he could have stood in their place and have completed the mission they had not finished themselves. However, since Pinchas was already alive at that time when Nadav and Abihu died, and his soul was already within his body, how could they come into gilgul within him?

Rabbi Shimon replied: My son, this is a supernal mystery. Here is the explanation: When they Nadab and Abihu passed away from this world, they were not protected by the wings of the holy rock [an expression signifying the Shechinah]. Why not? Because, as it is written, "they had no children". (Num. 3:4) They had not fulfilled the mitzvah of "be fruitful and multiply" and therefore reduced in stature the Divine Image. One who has children therefore augments the Divine Image…

A human being is created in the Divine Image and Likeness. It follows that one who has children therefore augments the Divine Image, and one who does not have children, in a sense, reduces the Divine Image. Accordingly, they were not worthy of serving as High Priests and therefore should not have brought the incense offering into the Holy of Holies as they did. Pinchas acted zealously to protect the holy covenant sealed in a man's flesh through circumcision by publicly impaling the licentious sinners with his spear. But when he saw many men from the tribe of Shimon whose leader, Zimri, he had just killed advancing upon him, his soul flew out in fear.

Then the disembodied souls of Nadab and Abihu entered into him and became one. Subsequently his own soul also returned to him, and they all became one. In this way he inherited their positions, and he became a priest, which was impossible beforehand. This is called live gilgul, or ibur, as explained above. The advantage to Pinchas was that he became a priest and was given G‑d's Covenant of Peace. (Num. 25:12-13) He was worthy of this because of his passion for the sanctity of sexual relations that some Israelites and Zimri had violated. Now, through Pinchas, Nadab and Abihu were able to fulfill the mitzvah to be fruitful and multiply. (Zohar III p. 217a)

However, a question remains: Why did both Nadab and Abihu enter into Pinchas, instead of choosing different hosts? The following extract answers this question:

Rabbi Elazar asked his father [i.e. Rabbi Shimon bar Yochai): Nadab and Abihu] were two people, so why are they not found as two? [In other words, why did they both come into ibur in Pinchas instead of each finding a different host?] He replied, "Because they were each only two halves of a body, for they had not married [and a man who does not marry is regarded as being only half a person]. Therefore, the two were included as one, as it is written, "she bore Pinchas; these are the heads of Levite clans…" [in the plural, as explained above]. (Zohar III p. 57b)

The concept of "dibuk" is a concept closely related to ibur, except that whereas an ibur is beneficial to its host, a dibuk is harmful. "Dibuk" may be translated as "clinging". The disembodied soul of an evil person seeks out a vehicle for its wicked machinations. It therefore attaches itself to the soul of another person, hampering the latter's ability to act independently or even rationally. This could occur as a result of a persons transgressing the same mitzvah that the dibuk transgressed. This injurious form of possession typically would result in the death of the possessed, unless the dibuk was properly exorcised.