Kuji-in is the spiritual and mental strength the ninja possessed in the form of hand signs. These hand signs were believed to be able to channel energy. The hand signs were taken from the practices of the early Buddhists.
Kuji-in was used to build confidence and strength in the ninja. It was also believed to enhance the senses of danger and foresee death for the ninja.
In Kuji-in the thumb represents the source of power (Ku), and the fingers represent the four elemental manifestations. They are Chi (earth – solids), Sui (water – liquids), Ka (fire – combustion) and Fu (wind – gases).
There are 81 hand symbols, total, but there are 9 primary ones: Rin, Hei, Toh, Sha, Kai, Jin, Retsu, Zai, and Zen. Each symbol invokes different strengths and abilities in a trained ninja.
Rin brings strength to the mind and body;
Hei generates psychic power in order to mask one’s presence;
Toh enables the ninja to reach a balance between the solid and liquid states of the body, which leads to a greater harmony with the universe;
Sha is used to heal oneself or another;
Kai gives complete control over the body’s functions, enabling one to slow the heart rate, endure extreme heat and cold, etc.
Jin increases the mind’s telepathic powers, giving a highly trained Ninja the ability to read the character of another;
Retsu gives telekinetic powers, enabling a ninja to stun an opponent with a shout or touch.
Zai extends the harmony gained by merging with the universe.
Zen brings enlightenment and understanding.
Kuji-in takes an entire lifetime to truly master.