Eight Trigrams Sealing Style

The Eight Trigrams Sealing Style, consisting of two Four Symbols Seals, was used by Minato Namikaze to help Naruto Uzumaki access the chakra of Kurama, the tailed beast sealed within him.

Usage
To perform the technique, the user must first summon a ceremonial throne and place the target on it. The seal was not automatically visible on Naruto's belly, but became perceptible whenever he drew Kurama's chakra.

Effects
Whenever Naruto and Kurama conversed, it occurred within Naruto's subconscious, which was depicted as a basement of an industrial plant, with pipes running along the ceiling and the floor — the latter of which was flooded by water. On the entrance gate of Kurama's cage, there was a parchment of paper that said, which covered the lock that kept the fox in. Minato also sealed his and Kushina's remaining chakra into Naruto with this seal. Minato appeared in Naruto's subconscious when Naruto transformed into his eight-tailed form, and restored the seal to its original strength, but he could only do it once. Kushina also appeared and assisted Naruto when he later fought Kurama to control its chakra.

The seal itself would later evolve during Naruto and Kurama's fight, becoming a new, stronger seal that consisted of massive torii that was capable of imprisoning the fox with ease after Naruto separated it from its chakra and merged with his own. The new seal's gate has the same lock as its previous one.

Key
Because the seal itself would naturally weaken over time, Minato, before his death, created the key, on the abdomen of the scroll toad Gerotora, that can either strengthen or completely remove the seal. He gave it to Jiraiya for safekeeping, with an intention to give it to Naruto by the time he would need Kurama's power to fight against Tobi.

Trivia

 * In the anime, Orochimaru suited Mecha-Naruto with the sealing technique in order to capture Kurama from Naruto.
 * In the original dub during the his fight against Zabuza, Kakashi, with his limited knowledge of sealing techiques, believed Naruto's entering his initial jinchuriki transformation meant the seal had almost completely broken but this was nowhere near the case.