Eight Trigrams Sealing Style

The Eight Trigrams Sealing Style is a fūinjutsu devised by the Uzumaki clan and consists of two Four Symbols Seals.

Usage
This technique was used by Minato Namikaze to help Naruto Uzumaki access the chakra of the Nine-Tails sealed within him. 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 on the fox's chakra.

In the anime, this sealing technique is specifically done to force a tailed beast to aid its jinchūriki by creating a small "crack" in the seal. By providing its chakra to the jinchūriki, the seal will gradually weaken, thus giving the beast a chance for its freedom. This is a potential risk to a jinchūriki as they only have so much time to become strong enough to subdue the tailed beast and obtain a stronger seal. To buy the jinchūriki more time, the seal can be locked completely, preventing the seal from weakening further, but also preventing the jinchūriki from accessing more of the tailed beast's chakra.

Effects
Throughout most of the series, whenever Naruto and the Nine-Tails 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 the Nine-Tails 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 the Nine-Tails to control its chakra.

The seal itself would later evolve during Naruto and the Nine-Tails fight, 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 created a key before his death that can either strengthen or completely remove the seal. He left the key on the abdomen of the scroll toad Gerotora and sent it to Jiraiya for safekeeping, with an intention to give it to Naruto by the time he would need the Nine-Tails power to fight against Tobi.

Trivia

 * In the anime, Orochimaru suited Mecha-Naruto with the sealing technique in order to take Kurama from Naruto.
 * According to Killer B, this seal is higher in quality compared to the Iron Armour Seal.