I have put some time recently in researching this topic in the Internet and in some books (so everybody are free to not believe me - this is not a kind of stuff I have seen with my own eyes, let alone received any training on), so I just wanted to write this as a small addition to the article.
There are three mudra (hand seal) systems I have read about:
1. kuji-in (nine-symbol seals) is a set of 9 mudras (it is said that there are totally 81 seals, but only 9 were chosen as kuji-in) which, as far as I understand, deal with energy channeling, associating fingers with different organs and energies. There is also a concept of tenth symbol in kuji-in, which is to be thought of by the practitioner himself to combine the effect of all kuji-in seals. Kuji-doshin-ho is an application of Kuji-in, which adds mantras (Rin-Pyo-To-Sha-Kai-Jin-Retsu-Zai-Zen) to the seals, to form techniques of protection using nine syllables.
2. kuji-kiri (nine-symbol cuts) is a complex technnique involving mudras, mantras and possibly mandalas, but there is not much known about this. The stuff available online is mostly either false or oversimplification, and does not reveil the complexity of kuji-kiri. It seems that this is a truly secret technique and is likely to be known only by the ninjutsu Grandmasters themselves
3. Ketsu-in (probably translated as "seals of excellence") is a not-widely-known set of 5 mudras (of them I have only seen 4 in one book, and still have not found any pictures online) which deal with elementals, associationg one of Earth, Water, Fire, Wind and Void to each of the fingers respectively (starting with the small finger).
Choice is an illusion created between those with power and those without