Ibur occurs for two reasons. First, through the ibur of the righteous, the Nefesh of a person can become rectified to the level of the Nefesh of the righteous tzadik. In the World to Come he will ascend to that level since the righteous tzadik will have helped him to add mitzvot and holiness to his life. This reason serves the person [receiving the ibur] himself.
The second reason is for the sake of the righteous tzadik who was the ibur. By helping the other person to perform mitzvot and rectification, he gains a portion in them. This is the sod [meaning "secret"] of what Chazal wrote: Great are the righteous people, for even in death they merit children (Sanhedrin 47a). In other words, when they cause the person to increase his merit they become like "fathers" who guide and help. This is to his merit.
The righteous tzadik who has entered as an ibur is like a spiritual, internal navigation system
The righteous tzadik who has entered as an ibur is like a spiritual, internal navigation system for the host person. For this service the tzadik will also receive reward through the mitzvot he is helping the person to perform and the holiness he is helping to increase.
The righteous soul which enters a person and assists him during his life, as an ibur and not as a gilgul, "easily attains reward and is distant from loss."
This is an expression used in the Talmud regarding the laws of monetary investments and interest, borrowed here to describe the secure benefit accruing to the righteous soul in the circumstance of ibur.
Every time the person does a mitzvah, he [the righteous soul] receives reward. This is the secret of what is written, "A righteous person receives his reward and the reward of his friend in Gan Eden" (Chagiga 15a). Understand this deep secret well, though now is not the time to discuss it at length.
The righteous person receives the reward for having performed mitzvot during his own lifetime, and for those he helps his "friend" perform while an ibur inside of him.
On the other hand, if the [host] person does evil, the righteous soul will not suffer any punishment or loss since he only came to help him, not to cause him evil. On the contrary, if this person negates what he rectified, then the righteous soul will leave him.
Thus, the righteous tzadik "easily attains reward" for his assistance in the performance of mitzvot. At the same time he is "distant from loss" because he will not suffer any detriment or pain from the sins of the host nor need to participate in the host's sufferings.
If the person continues to act righteously, then the righteous soul will remain
The sod [meaning "secret"] of ibur is that it occurs during a person's lifetime and therefore does not bind [the Nefesh of the righteous tzadik] to the body. This is unlike a Nefesh that reincarnates, that enters as a gilgul from the time of birth and is unified and bound with the body without exiting from there until death. On the other hand, the Nefesh of the righteous tzadik enters as an ibur willingly and leaves at will. If the person continues to act righteously, then the righteous soul will remain there in order to receive a portion in the actions of the person. It will remain there until the host dies, when they will both go up to the same place, as mentioned previously.
However, if the person does evil, then the righteous soul will become disgusted with this union and leave it. He is not permanently there, but "borrowed," like a guest who remains with his host until he feels it is time to leave.
For the same reason the righteous tzadik will not feel any pains that may occur to the host person and he does not have to suffer them, because he is not "attached" to the body; he is only "borrowed."
His ruach and neshama will be worthy to be clothed by this nefesh
The rule is that a person who performs a mitzvah of great importance can merit an ibur of the Nefesh of a righteous tzadik from an earlier generation. As a result, it is possible [for him] to become rectified and purified to the point that his Nefesh will actually become transposed onto the same level as that of the righteous tzadik.
Afterwards, the person will have to rectify his Ruach and Neshama to such an extent of purity that they will be worthy to be clothed by this Nefesh.
The word translated as "to be clothed" indicates the kabbalistic concept of enclothement, hitlabshut, and comes from the root that means "to wear," as when it is said, "He wears warm clothes," or "He wore a new suit." Derived from it, for example, is the Hebrew word levush, which means "garment" or "clothing."
The concept of hitlabshut is actually a derivative of the concept of partzufim, both of which are among the most important and innovative ideas in the Lurianic Kabbala. (For a basic article on these innovations, click here).
Hitlabshut refers to the fact that the partzufim, or parts of the partzufim, are ensconced or enclothed one within the other, or dressed one upon the other. (For more on hitlabshut, see subsequent articles)
Then he will actually become transposed [in his entirety] onto the level of the righteous tzadik, having ascended beyond the original root-level of his soul from whence he came. All of this is the reason for the help and assistance of the righteous soul.
[Commentary by Shabtai Teicher.]