There is another difference between yibum and gilgul, which was mentioned at the beginning of this analysis. If a person reincarnates in the secret of yibum, his first body is considered as if it had never existed at all. For this reason, the entire Nefesh returns as a new creation. It is possible, therefore, that the Ruach and Neshama can reincarnate together with the Nefesh, though not all at one time.
When he merits and performs mitzvot fitting for the Ruach, then it will enter him; and it is the same with respect to the Neshama. About a person who came into the world for the first time, the Sabba of [Zohar] Mishpatim explained, "If he merits more, they bring him a Ruach; if he merits more, they bring him a Neshama, etc." This is not the case, as we have explained, for someone who reincarnates.
The one who has come back in the secret of yibum is as if he has come into the world for the first time, and all the rules apply accordingly. This is the case even though it is the second body of his Nefesh (and perhaps it is even a later body of a subsequent gilgul after a second or even a third reincarnation).
The brother-in-law has the ability to return a portion of the nefesh of his brother into this world through yibum.
Therefore, someone who returns in the secret ofyibum is similar to a new creation, and he is able to achieve his Nefesh, Ruach and Neshama (NR”N) in one gilgul, if his actions warrant it, as was mentioned. This is the esoteric meaning of the verse, "If he sets his heart upon it, then he would gather into himself his Ruach and his Neshama" (Job 34:14). This is understood in Sabba of [Zohar] Mishpatim in a deep way to refer to one who has returned in the secret of Yibum.
The simple description of the mitzvah of yibum is as follows: The wife of a man who has died without leaving any progeny in the world, will marry the brother of the deceased. This latter is called "the brother-in-law". The child that is born from this union of the wife and her brother-in-law will be called by the name of the deceased. It is considered as if the soul of the deceased, who has left no progeny, has come back into the world. The brother-in-law has returned a portion of the Nefesh of the deceased into this world.
Just as the brother-in-law has the ability to return a portion of the Nefesh of his brother into this world through yibum, so too can yibum itself collect and return to the Nefesh both the Ruach, and Neshama. However, it must be dependent upon good deeds, just as it says, "If he sets his heart upon it." (Ibid.)
The only reason it comes back is in order to help the ruach to do good
A gilgul, on the other hand, does not have the ability to draw all three parts of the soul to him, but only one at a time. As mentioned before, first the Nefesh reincarnates alone until it becomes completely rectified, after which the person dies. Next, the Ruach alone is born into a different body until it is rectified. The Nefesh will also reincarnate together with it, but only as an ibur, since it is already rectified. The only reason it comes back is in order to help the Ruach to do good, and not bad. Therefore, it receives a portion of the good deeds of the Ruach, but not the bad deeds, just as we said with respect to the Nefesh itself when rectified sparks return in the secret of ibur.
This is another explanation of how there can be an end to the reincarnations of the Nefesh, for it has no part in the sins of the Ruach, as mentioned.
Once it is rectified, it remains rectified no matter what the un-rectified Ruach or Neshama do while in the body.
After he dies and the Neshama reincarnates to become rectified, then the Nefesh and the Ruach reincarnate only in the secret of ibur, until it is purified. After that, there will no longer be any need for this person to reincarnate into this world for his own sake. However, he may return into this world as an ibur in another person, during that person’s lifetime, to assist him and thereby receive a portion with him, as was explained previously at length.
[Commentary by Shabtai Teicher.]