From this, we understand that when a Nefesh reincarnated into this world (and the rectified sparks come into the body, but in the status of an Ibur, but perhaps they can be benefitted because of a minor sin that was previously committed), the purpose of the gilgul of the Nefesh [is not for the sparks] but it is for the specific part of the Nefesh that he damaged (through a serious sin). So the sparks that come with him help the body and the other parts of the Nefesh (because they were already rectified in other bodies this person already had, and they were rectified then).
Thus, the nitzutzim (sparks) do not enter the new body except with the status of an Ibur. Thus, when the part of the body [that was previously involved in a serious aveira in a previous body] does whatever mitzvah it needed to in this world (in this gilgul), so too, all the parts of the Nefesh that came into the body with the status of an Ibur also benefit from the mitzvot this person does in this body.
Not only this, but the person in this new body also helps (through his other mitzvos) the nitzutzot which come into his body (since they can benefit from mitzvot which are not the reason he reincarnated into this body [just as the secret of an Ibur can help a tzaddik who enters into his body to help him in the form of an Ibur]). However, if he sins, these sparks or any other parts of his Nefesh that have already been rectified are not punished because they are only there to help him for good, and they are not affected by the bad he does:...