We must explain the reason for what we said, which is, because Asiyah is the lowest of all of the worlds, the Klipot (impure forces) surround him. Thus, even though an Adam (a person) has rectified his Nefesh, and even though the source of that Nefesh might be at some lower level of Asiyah, nevertheless, if the soul was left at that lower level [when it was rectified], we would be concerned that maybe the Klipot would grasp onto it (because the Klipot are there).
Thus, a person needs to merit from his actions more and more, until the source of his Nefesh is in Keter of Asiyah (I assume where the Klipot cannot reach it). However with Yetzirah, and so too the other worlds above it, there is no fear that the Klipot will take hold of the soul as it would in the world in Asiyah. Therefore, when one rectifies the soul which has his source in Yetzirah, even if he rectifies more [a soul on a higher level], that first sould stays in its original place, and he receives a second Ruach which is at a higher level, and the first Ruach does not need to elevate higher, because at that level there is no concern:...