[Need a better translation in the following section.] We also must know what is happening when the Nefesh enters into a gilgul without his Ruach, as we described. The secret of the concept is that according to the level of merits and elevation that this person's Nefesh received, so too, into that same level he will reincarnate. However, in that same body of the person while he is still alive, he can also receive another Nefesh of a Tzaddik that already completed his Nefesh (and does not require gilgul).
This Nefesh of a Tzaddik can enter into his body (via the secret of Ibur), and this Nefesh can come in the place of where a Ruach would ordinarily go [noting that this person's Ruach cannot join him in his lifetime, so the Nefesh of the Tzaddik 'fills the emply slot' where the Ruach would go]. This second Nefesh helps the person perfect himself more fully. Sometimes it is possible that Nefashot of Rishonim, even of Avraham Avinu (alav haShalom), and others like him [they could enter into him as an Ibur] in order to rectify and purify the Nefesh of this person.