Ibur takes place during a person's lifetime, as we have already mentioned.
In Chapter 2, Section 3 it was written: "…There will reincarnate into the body of this person, while he is still alive, the Nefesh of a righteous tzadik." This is called ibur, and more specifically, "ibur while alive." The difference between this type of ibur and gilgul was also explained there.
Normally, ibur takes place during a person's lifetime; it involves parts of soul that come to a person years after he is born. Gilgul, on the other hand, involves parts of soul that a person is born with. They reincarnate from the time he is born, and they stay with him until the end of his lifetime.
(Later on in this chapter, there will be introduced a new concept of "ibur from birth," and it will be explained there, God-willing, when we come to it. The subject of this section, however, is the normal type of ibur.)
Sometimes a certain mitzvah may come before a person, and he performs the mitzvah as it should be done. At that point the Nefesh of an earlier righteous person who had performed this same mitzvah correctly will join the person as an ibur, since they are alike with respect to this mitzvah.
Thus, the actual mitzvah to be performed affects which righteous soul will join a person as an ibur.
Now we are learning that the ibur can even come from a living righteous individual
Not only this, but it is also possible that the righteous person is alive during his lifetime, and still ibur can occur.
The one who performed the mitzvah and the righteous person are alive at the same time, and still ibur can occur.
Thus, if a person performs a particular mitzvah or mitzvot relevant to a righteous individual who also performed correctly, then the Nefesh of that righteous tzadik can enter the person, even while they are both alive at the same time.
Up until now we knew that ibur occurred in a living body after birth, but with the soul of a righteous person who has already died. Now we are learning that the ibur can even come from a living righteous individual.
This is the secret meaning of the verse, "The Nefesh of Jonathan became attached to the Nefesh of David" (I Samuel 18:1). In other words, even while they were both alive, the Nefesh of David was joined with Jonathan as an ibur.
This accounts for the strong soul bond they had while they were alive.
(Rabbi Chaim Vital, who recorded these works, says: It appears that one mitzvah performed correctly is enough to initiate the ibur. It is not required that he have performed all mitzvot to this point).
[Commentary by Shabtai Teicher.]