Demonic Phoenix
Senior Member
I'll explain this as best as I can, so bear with me. I'm not saying that it is correct; rather it is how I understand it.
What do we know?
We already know that Minato split the Kyuubi's chakra into two parts, Yin & Yang.
We also know that there are two seals on Naruto. One is the Shiki Fūjin, the other is the Eight Trigrams Sealing Style.
To put it simply: The Yang chakra is sealed using the Eight Trigrams Sealing Style. The Yin Chakra is sealed using the Shiki Fūjin.
Onto the explanation:
a) The Eight Trigrams Sealing Style allows Naruto to access the Kyuubi's chakra, in this case, the Yang half of the Kyuubi's chakra.
b) The way the Shiki Fūjin is shown to work in the Sarutobi-Orochimaru fight is this:
1. Shinigami grabs opponent's soul.
2. Shinigami seals opponent's soul within caster's body. Characteristic seal is left on the caster's stomach. (Caster being the person using the Shiki Fūjin)
3. Shinigami then takes the caster's soul as the cost. If the opponent's soul is sealed in the caster's body, then the opponent's soul is taken as well.
(Read Chapter 123-124 for proof)
c) This is a common point that many overlook:
The reason the Shinigami did not get the Yin half of the Kyuubi's chakra should be clear based on Steps 2 & 3.
Minato never sealed the Yin half inside himself, he somehow sealed it inside Naruto. The Shinigami never took Naruto's soul, it took Minato's, as he was the 'caster'. Therefore, the Shinigami never got the Yin Half.
d) What does this mean? It means that Naruto can access the Yang half due to the Eight Trigrams Sealing Style. The Yin Chakra is not accessible because it is sealed using the Shiki Fūjin. This also means that the entirety of the Kyuubi's chakra is sealed in Naruto, but he can only access the Yang half.
tl;dr - The entirety of the Kyuubi's chakra is sealed within Naruto, but he can only access the Yang half due to the nature of the seals used to seal the two halves.
Therefore, Madara must have a way to get past the Shiki Fūjin seal. Since the Yin half of the Kyuubi's chakra is sealed within Naruto, and not within the Shinigami, it functions like any other Fūinjutsu.