The Truth About Minato's Shiki Fuujin No Jutsu and the Kyuubi's Yin and Yang Chakra
Here's what I found on Net:-----
link:-The Truth About the Kyuubi's Sealed Chakra and Naruto's "That" Jutsu - Naruto Forums
STATUATORY WARNING:-It is way too long..................
Edited to fit current scenario...........Please tell me if my editing(though very small), is wrong..............
We all know that although the seals that were made by the Shinigami variation that Sarutobi used, and the Shinigami variation that Minato used.. are still the SAME jutsu.. they involve the same process... but DIFFERENT ways on how it was performed by Minato and Sarutobi, respectively. In a sense, the jutsu is a Kami Fuujin Jutsu [God Sealing Jutsu] a level of Fuujin jutsu so powerful, that it requires a GOD to perform and a sacrifice. I am pretty sure its clearly stated, that Sarutobi just about uses the same jutsu. Take a look into the scenarios... I will start with Sarutobi's...
Faced off against an opposition he cannot defeat, and wanting to end the idea of having Nidaime and Shodaime ever hurt Konoha again along with Orochimaru. He uses the ultimate sealing jutsu that he knows. This jutsu, causes the user to go into the belly of the Death God and any other souls that are ingested in the process. The Nidaime and Shodaime souls were pulled out, and placed into Kage Bunshin. That being apart of his chakra now, they were both eaten in the process along with Orochimaru's arms. Sarutobi died, and had a new seal upon his body that was similar to Naruto's.
Faced off against an opponent, nobody could ever dream of defeating he figured of using the most powerful Fuuin jutsu he could harness. A jutsu that was developed in case this kind of scenario would happen. Instead of, the Shinigami reaching through a clone of Minato... it reached through Naruto. But, for this sealing it was a variation of the first seal. The combination of seals... the Double 4-Symbol Sealing Technique seals the Kyuubi into Naruto's system and prevents it to escape. The seal forms the 8-Trigram Hakke Sealing Style allows Naruto to tap into the Kyuubi Chakra making Naruto's body an ultimate chakra machine. However, there is a weakness with this seal as it can be weakened eventually. Its a possibility that the Kyuubi could break the seal over time, and manage to find his way back into the world.
The Final Sealing:
Minato used the SAME jutsu that the 3rd used against Oro, the Dead Demon Consuming Seal. The 3rd himself CONFIRMS it.
The Kyuubi is to be killed ONE WAY, by sealing it in a newborn and that jinchuuriki (human sacrifice) dying. Now, when we say the SAME jutsu it doesnt necessarily mean that Minato and Sarutobi did the same STYLE of sealing. So the question is, HOW did the Kyuubi ended up being sealed INSIDE Naruto instead of being consumed by the Death God?
How did Minato do it? What exactly happened ?
Minato's Shiki Fuujin No Jutsu and the Kyuubi's Yin and Yang
Here's how Minato did it:
1. He summons the Death God invoking it's power
2. The arm of the Death God reaches through Naruto's body
3. Death God pulls the Kyuubi towards Naruto's body.
4. Minato divides the chakra into Yin and Yang, sealing the Yin chakra inside the Death God, and the Yang chakra inside Naruto. He uses the Double 4-Symbol sealing jutsu to seal the Kyuubi inside Naruto, forming the 8-trigram Hakke sealing style, allowing some of the Kyuubi's chakra to mix with the kid's chakra.
Minato splitted the Kyuubi chakra into Yin and Yang so that the Death God would have the 'shadow' (Yin), and Naruto would have the 'light' (Yang). He did this for two reasons: (1) to seal the Yin chakra of the Kyuubi inside the Death God's belly, leaving the Yang chakra to be sealed inside Naruto, AND (2) to leave his son and his village a powerful weapon.
5. Death God pulls his arm out of Naruto, leaving a spiral symbol on Naruto's stomach, which looks exactly the same as the spiral which appeared on the Third's stomach after performing the technique
6. Since the jutsu requires the soul of the user, the Death God consumes Minato's soul in the process.
7. Kyuubi is sealed and Minato dies.
The Mystery of Minato's Death: What Really Happened
This is the mystery surrounding Minato's death. We all know how Sarutobi did it, but didn't saw Minato do it. Since Sarutobi says Minato used the SAME technique in sealing the Kyuubi, i have came up with this detailed explanation on how he performed the Shiki Fuujin no jutsu.
Simply said, Minato probably was FIGHTING MADARA and Naruto was being born when Kyuubi attacked. Konoha ninjas tried desperately to stand their ground, until Minato comes.
Minato defeats Madara but Madara escaped(We can say, even if 4th said, Tobi saw through his moves). Minato heads out to Kyuubi, summons Gamabunta, and takes his chakra-filled ass for a ride. Now, knowing his son was soon to be born, and knowing he had no time and that he had to seal Kyuubi in a child that was just born or not born yet, he has Gamabunta or some one else take his child. He puts Kyuubi in his kid just after the cord was cut, and then places the Double 4-Symbol seal forming the 8-Trigram Hakke seal on him with the last of his life and chakra. He dies after sealing the Kyuubi, as did the woman who died through grief of her loss and the stress all at once. The baby has no name, but a swirl on his stomach. Naruto, if im not wrong, means spiral, or swirl. And theres also an ingredient in Ramen named that? Jiraiya's main character in his book is named Naruto, and before the child was born his parents agreed to name him after Jiraiya's main character in his novel. Maybe Minato liked Ramen a lot too, and in honor of Minato's will, and the things Minato loved, they named him Naruto.
Minato's Power and His Legacy
Now, people may get iffy about... how did Minato have that much chakra to spare... as Sarutobi didn't. Minato, we dunno how much chakra he had. However, at this point the only thing he knew with that battle is... he summoned Gamabunta. That does take an enormous amount of chakra, but that is a lot more chakra than Sarutobi had during his point he used his jutsu. I mean, Sarutobi had been battling and using up his chakra against Oro [Shuriken Kage Bunshin, Katon: Karyuu Endan (Fire Dragon Napalm), Doton: Doryuu-Heki (Mudslide Barrier), Ninpou: Kuchiyose No Jutsu] And after all that, he split his chakra into thirds. Him, and his two kage bunshin each having a third of his current chakra. And that was enough of a sacrifice, to get the souls of the Shodaime and Nidaime out with no problem.. but he did not have the power to subdue Orochimaru entirely... now, if one third of Sarutobi's chakra after all of that was enough to seal Two Kage Level individuals whom have a great deal of chakra.. and the arms of a Sannin... Think how much chakra would be needed to extract the Kyuubi? Minato himself could.. hold back the Kyuubi's power, thats a great wealth of power in itself.
The Shiki Fuujin Jutsu, the one that both Sarutobi and Minato used is a God Sealing Jutsu. Just like how there are individuals whom have specific jutsu relating to their summons. We have seen jutsus that are involve with the summons already, I dont see why it would be far fetched to have a jutsu that is pretty much already explained. Dealing with the Shinigami extracting a soul, and sealing the soul, at the cost of the users life. How do they knew it will work? Cause they can summon that creature, they have a pact with that creature. When you have a pact with that creature, you most likely have some form of communication with them at some point whether it be spiritually or physically. Like selling your soul to the devil to get the one thing you desire, this thing being.. the sealing of the Kyuubi to create a potential tool that will be Konoha's greatest asset.