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Although it is Hao's written book, it is a powerful concept in shaman king. It boosts the abilities of shamans who understand the concept of Ultra Senji Ryakketsu and Shamanic Oracle.
Overview
In volume 11, Yomei Asakura introduces the Ultra Senji Ryakketsu to Anna. He originally wanted himself to bring the book to Yoh, but after witnessing Anna using her 1080 beads to subdue Hao's familiars/ ogres, he decides to make Anna teach the Ultra Senji Ryakketsu to Yoh Asakura.
In volume 12, Yoh Asakura and his friends arrive to Patch village and at the same time, Anna, Manta, Mikihisa, and Tamao (w/ her spirits) arrive, too. Due to new tournament rules and the second tournament is coming in two months, Yoh has to make a three shaman team. Anna decides to train Yoh and his teammates, Bokuto no Ryu and Faust the VIII.
In volume 15 (after a time skip of two months), Yoh, Bokuto no Ryu, and Faust the VIII display their amazing new found abilities. Yoh can use a compound oversoul, using two mediums and integrate a serei spirit or higher twice. Both Yoh, Ryu, and Faust have extremely large oversoul that use up a lot of mana.
Hao Asakura attempts to recruit Ren Tao when he tells Ren, if he joins Hao's group, Hao will teach Ren his updated Ultra Senji Ryakketsu. Ren refuses Hao's offer and says he can get stronger by his own way. Hao mocks Ren when Hao tells Ren that he cannot defeat Yoh or Hao.
Abilities and the Shamanic Oracle
Increase mana leads to bigger oversoul
Yoh, Ryu, and Faust demonstrates this in volume 15.
Increased mana allows one to integrate creatively
Yoh demonstrates a new way of integration called the compound oversoul. This term is coined by Hao Asakura as he explains to Opacho how one creates a compound oversoul.
Shamanic Oracle
According to Nichrome (in volume 17), "If you can discern the changes in an

Shamanic Oracle
enemy's mana, you can use that knowledge to guide your attacks."
What he means is that a shaman can hone one's abilties to detect mana and ride the wave of mana in order to dodge quickly.
Examples: Mikihisa who can create illusions and dodges every move from Ren. MIkihisa tries to teach Ren through battling.
Ren quickly learns the shamanic oracle when he dodges Mohamed and Zang-Chin's moves and counterattacks with ease.