There are two types of gilgulim and two types of ibur.
The first type of gilgul occurs when a single Nefesh enters the body of a person at the time of birth.
The second type is also possible. Two Nefashot may reincarnate together, and this is also at the time of birth. This is called "Double Gilgul" [Gilgul Kaful]. It was explained in the previous chapter, and in other places.
Both these Nefashot [in Double Gilgul] reincarnate and come into the world together when the person's body is born. They do not separate until death. They are called one Nefesh. As one, they suffer the pain and punishments that are inflicted on the body throughout its lifetime, as well as the pain of death.
Ibur, however, does not occur at birth, as we explained earlier, and there are two types. The first occurs for the benefit of the righteous tzadik himself who enters a person to complete himself with something that was missing to him [the tzadik]. The second type is for the sake of the [host] person, to assist him with Torah and mitzvot.
Thus, gilgul takes place at the time of birth, and the two types are:
1) One Nefesh reincarnates into a body.
2) Two Nefashot reincarnate into the body (Double Gilgul).
The two types of ibur are:
1) The soul of a righteous tzadik comes into a person because the tzadik himself is missing some tikun. This ibur is for the benefit of the tzadik.
2) The Ibur is for the benefit of the host person. The soul of the tzadik has only come to help him.
Ibur does not take place at the time of birth, as the Rav will explain now. Then he will go on to further clarify some aspects of ibur.
the soul of the tzadik enters and spreads throughout the entire body, just like the person's own Nefesh
When he [the soul of the tzadik] comes for his own sake, then he does not enter the person until he is thirteen years old and one day. At that time the (host) person becomes obligated in Torah and mitzvot. In this way, he [the tzadik] can also rectify himself through this person, and that is why he does not enter before this time, only after the obligation of mitzvot has taken effect.
At that time, the soul of the tzadik enters and spreads throughout the entire body, just like the person's own Nefesh. The two of them suffer all bodily pain together and equally. It remains there for a set time to rectify and complete that which it needs. Then, it leaves while the person is still alive, and returns to its place Above in Paradise.
This completes the description here of the first type of ibur, where the soul of a tzadik comes for its own benefit to complete some tikun that was missing to it. The Rav has added here that since the soul of the tzadik is coming for itself, even though it is coming as an ibur, it also suffers the pains of bodily existence. It gains the merits of the Torah and mitzvot performed while it resides in that body. It does not suffer the obligations of any sins that may be committed during its residence in the body, as we learned previously. It only comes when the host is obligated Torah and mitzvot, and it leaves when it has finished with its own tikun.
The second type of ibur… comes in order to help the host perform Torah and mitzvot…
In the second type of ibur, discussed now, the soul of the tzadik comes for the benefit of the host and does also not arrive until the age of thirteen years and a day. It comes in order to help the host perform Torah and mitzvot; thirteen years and a day is the time when that assistance becomes useful.
However, when [the soul of the tzadik] comes for the sake of the person and not for himself, then it comes of its own volition and not by coercion. Therefore, it is not forced to suffer any bodily pain at all, and it does not feel whatsoever the sufferings or afflictions that come upon [the host's body]. Furthermore, if he is pleased with the host person, then he remains; if not, then he leaves him, as it says, "Leave the tents of these evil people…" (Numbers 16:26).
[Commentary by Shabtai Teicher.]