The Eight Trigrams Sealing Style is a fuinjutsu of the Uzumaki clan. It is based on the Four Symbols Seal, designed to imprison a large enemy or an evil spirit within an object or within a human body. If two such seals are applied, a more powerful barrier is produced - the Eight Trigrams Sealing Style. When the seals are placed not completely overlapping each other, the chakra of the sealed creature can mix with the chakra of the host.
Minato Namikaze used this technique to seal the Nine-Tails within his son Naruto. Before the ritual, he summoned a special altar and placed the infant upon it. The seal was located on Naruto's abdomen, but manifests only when Nine-Tails chakra is used. As additional protection, Minato sealed inside his son the chakra of both himself and Naruto's mother, Kushina Uzumaki. They could appear under certain conditions: Minato when the Fox nearly convinced Naruto to remove the seal, and Kushina when he tried to replace the seal with a new one.
Over time, the seal naturally weakens. In order to be able to reinforce it, Minato created a key and wrote it on the belly of Gerotora's toad scroll, then passed it to Jiraiya. Jiraiya, believing that Minato wanted Naruto to use the Fox's power, slightly weakened the seal, giving Naruto greater access to the chakra. Later, the key passed to Naruto himself, and he used it to remove the seal and replace it with a stronger one after defeating Kurama during training to master his power.
