However, in the Gilgul that takes place not through Yibbum [e.g., through a person blemishing his Nefesh via a certain serious aveira], he does not have the power or ability to bring down all three of them [the Nefesh, Ruach, and Neshama in one lifetime], but rather, one at a time [one more in each lifetime], just as we described. This is, in the first lifetime, he gets only the Nefesh until he completes it entirely and then he dies.
Then, he is reincarnated with a Ruach alone in another body until he completes it and then he dies, and then the Nefesh can reincarnate with the Ruach. However, with the secret of Ibur, when a soul is completed, it comes into the body only to help it (it cannot be damaged itself by the sins the person commits). And, since it comes into the body having the status of an Ibur, it takes part in the good deeds that are done with the Ruach, and not the bad [it is not affected by the aveiras the body commits]. [Please add translation.]
Also with this, we explained, how there is an ending to the reincarnations (gilgulim) of the Nefesh, and a person is able to perfect it, since it is not affected by the sins that are done by the person (who is working on perfecting the Ruach), as we explained. And then he dies, and then he reincarnates with his Neshama with the purpose of perfecting it, and the Nefesh and Ruach come with him but have the status of an Ibur, until the Neshama is perfected.
Then, there is no more need for gilgulim in this world for the perfection of his soul. However, he is able to come with the secret of Ibur into this world again in another person who is living -- to help him in his merits, and to give a piece of himself, as we described at length (in Ch. 2):