The Medrash (Medrash Tehillim 19:14) explains the verse (Tehillim 19:8): "Hashem's Torah is complete; it revives the spirit", by saying: "Why is Hashem's Torah complete? Because it revives the spirit."
The Noam Elimelech explains that, as mekubalim say, Korach was a reincarnation of Kayin. His being swallowed into the ground was a tikkun for how the blood of Hevel was soaked into the ground after Kayin killed him.
He asks why Kayin couldn't have this tikkun performed on himself while he was still alive and explains that Hashem had him return in a new gilgal as a means of making His Torah complete, as if Kayin had this tikkun done on himself, the Parshah of Korach would be missing from the Torah.
Thus, the Medrash is saying that Hashem's Torah is complete because Korach "revived the spirit of Kayin" – meaning that Kayin's spirit was reborn in him, and this completed the Torah.
