From what is said..
I understood that Minato used a combination of seals not only the death god seal, he used the constelation and the 4 seasons too..
Then he put all of kyubii chakra inside naruto, and sealed the YIN part away from naruto, that is represented by the gate with "seal" on. The Yin chakra is the kyuubi itself he can not pass through the gate but the "Yang" chakra of him, can.
That is what been seeping into naruto, and he can call upon.
The Key is used to "twist" and open the seal(the gate) so he can tap into the YIN part of the Kyuubi chakra.
"Chile is a thin and tall country"