Oh...my god. Can you imagine Yin and Yang being the AoE/ST toggle from 1.0's THM? AoE Huton anyone?
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It's just something made up for the game. The real Mudras have more than just 3 hand seals and they don't relate to Heaven / Earth / Human or those 12 animals like in Naruto. Each sign is a word and when performed in a sequence will create a sentence - like a hand-sign language, really.
Why stop at 4 mudras?
http://youtu.be/f0iAXdP8eX0?t=3m21s
The real practice of Ninjutsu and use of kuji-in is for meditation purposes. It is based on Esoteric Buddhist beliefs (Mikkyō).
Their meaning in relation to Ninjutsu.
Rin - Strength
Pyōh - Channel
Tōh - Harmony
Sha - Healing
Kai - Sense of Danger
Jin - Reading of thoughts
Retsu - Control of space and time
Zai - Control of sky and elements
Zen - Enlightenment
Mudras are popular and very easy to adapt with meditation and rituals. Which is why there are many different kuji-in / mudra across different beliefs and philosophies and have different meanings.
Video games, movies, and modern mythology obviously associate them having magical powers.
I don't watch Naruto, however since you mentioned animals, it is likely based off Taoist beliefs.
I can't think if the ability off the top of my head but the player is healed by ninjutsu many times during their Job Battles. its shayatoki or something like that. (really didnt pay attention to the spell name flying pass me as I was trying not to die, lol)
It's not a jutsu it's SuiYaku ( Sui 水 = water, Yaku 薬 = Medicine ) so a potion ( yes a convenient way to make something look cool, write it in japanese ! xD )