[Rough Translation] It could also be that during his reincarnations, it is not required that his Nefesh reincarnate with the Ruach during those gilgulim in which he is working to rectify the Ruach. The Nefesh can remain above in its place in 'Tzarur HaChaim', and the Ruach alone can go down into a gilgul to rectify itself. Here, a Ruach cannot come down on its own, except by dressing itself into [another] Nefesh.
If this happens, here, the Ruach dresses itself into the Nefesh of a Ger (as we explained in בסבא דמשפטים, and it reincarnates alone (without its Nefesh, but together with the Nefesh of a Ger), until the Ruach is rectified. Then the person dies from the world, and he returns in another gilgul. But, the Ruach that helped this person with his Tikkun of this person's Nefesh reincarnates together with him in order so that this person can also receive his Neshama (and then it helps him rectify his Neshama as well).
It could also happen that his Ruach enters into a gilgul with his Neshama [without his Nefesh] until his Neshama is rectified (completed). After the Neshama is rectified, the person no longer needs reincarnation, and the three of them (his Nefesh, Ruach, and Neshama) rejoin together above in 'Tzarur haChaim'.