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 the Nine-Tailed Demon Fox sealed within him. To perform the technique, the user must first summon a ceremonial throne, and place the target on it. Apparently, the seal was not automatically visible on Naruto's belly, but became perceptible when the latter drew chakra from the Nine-Tails.

Whenever Naruto and the Nine-Tails conversed, it occurred within Naruto's subconscious, which was depicted as a basement of a boiler room or a sub-level of a power plant, with pipes running across the ceiling and the floor being flooded over with water. On the entrance gate of the Nine-Tails' cage, there was a parchment of paper that said, which covered the lock that kept the Nine-Tails in.

It was later revealed that it was also designed to hold an imprint of this seal's user (as well as the chakra and personalities of other people that the sealer chooses) as a fail-safe. 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 Uzumaki also appeared and assisted Naruto when he later attempted to control the Nine-Tails.

Key
Because the seal itself would naturally weaken over time, Minato created the key, on the abdomen of the scroll toad Gerotora, that can either strengthen or completely remove the seal.

Gerotora explained that during Naruto's training, Jiraiya would normally use the key to suppress the Nine-Tails when necessary, but Jiraiya instead used it to weaken the seal for just a short time, so he could have Naruto try to filter out the Nine-Tails' hatred when he tapped into its chakra, but it instead unleashed the four-tailed form.

During Naruto's training to control the Nine-Tails at the Falls of Truth, the seal itself was destroyed when the Nine-Tails used its Tailed Beast Ball to try and blast Naruto after he used the seal's key to open the seal's gate. However, after the Nine-Tails' chakra merged with Naruto, he was able to imprison Nine-Tails itself with a new powerful seal that consists of massive interlocking torii.

Trivia
When Orochimaru used the Five Elements Seal on Naruto, this created an imbalance due to an odd-numbered seal being placed over an even-numbered one. As a result, Naruto not only lost access to the Nine-Tails' chakra, but also had difficulty controlling his own, until Jiraiya removed it.