Kuji-in is a meditative process of certain hand postures (called "mudras"), used to awaken the higher, spiritual self and join it with the physical. The phrase "kuji-in" is Japanese, but this practice has its roots in Hinduism. You may know some of the mudras if you know the show Naruto, and if you saw that new movie, Ninja Assassin, as they are usually associated with ninja (PLEASE note that Kuji-in is not like it is shown in either reference). To quote the Wikipedia page (linked at the end of this passage): "The Kuji-in practice symbolizes that all the forces of the universe are united against evil; because of this, it was often used by the common people for luck when traveling, especially in the mountains." They are also connected with the chakras, which I'm not going to list here because I'd like if people actually read the literature I will reference.
I personally like to use Kuji-in before and sometimes after meditation, as a way to center and focus. Don't be fooled by how simple the postures look; a few of them can be a (painful) workout for the hands, so it never hurts to stretch out the joints a bit before beginning. The sources at the end of this post contain pictures of the mudras and you can also check on YouTube. I wish there was a way to integrate pictures into the post; I apologize for the inconvenience.
Each mudra represents an idea or force, and each mudra has an accompanying mantra (which are supposed to be the original Sanskrit mantras put into Japanese phonetics). The interpretations of the mudras vary between Shinto, Buddhism, Ninjutsu. Ninjutsu is essentially a dead art, though several schools of martial arts claim to teach its techniques. My personal take is that you aren't a Ninja unless you kill people :3. You can either hold the mudra and repeat its name, or chant the mantra. You can do the full 9 or use them independently as you see fit, to draw on this energy. The Kuji-in and their respective mantras are:
1.) RIN: meaning strength and confronting your fears, your opponents, yourself. According to "Qi-Gong and Kuji-In" by Francois Lepine, RIN represents "accessing your power, energy, awakening your flame." I will include the interpretation from this book for all of the mudras.
The mantra in Sanskrit: Om vajramanataya swaha
The mantra in Japanese: On baishiraman taya sowaka
2.)PYO/KYO/HYO/HEI (just pick the one you like, lol): meaning perseverance, channeling of divine energies. It "circulates the energy in your body, allowing it to flow in and out."
The mantra in Sanskrit: Om ishaanayaa yantrayaa swaha
The mantra in Japanese: On isha naya in tara ya sowaka
3.)TOH: meaning harmony (conflictingly "to battle with"), and it "gathers energy in your bowels and circulates it."
Sanskrit: Om jitraashi yatra jivaratna swaha
Japanese: On jite rashi itara jiba ratano sowaka
4.)SHA: representing the foe or enemy, and it "distributes the energy to the necessary parts of your body, producing healing."
Sanskrit: Om haya vajramaantayaa swaha
Japanese: On haya baishiraman taya sowaka
5.)KAI: representing everyone/thing working together, and it "allows you to feel everything."
Sanskrit: Om namah samanta vajranam ham
Japanese: On nomaku sanmanda basaradan kan
6.)JIN: representing preparation of form and it "allows you to know everything."
Sanskrit: Om agnayaa yanmayaa swaha
Japanese: On aga naya in maya sowaka
7.)RETSU: represents moving forward and it "allows you to be aware of everything."
Sanskrit: Om jyotihi chandoga jiva tay swaha
Japanese: On hirota ki shanoga jiba tai sowaka
8.)ZAI: represents manifestation of the self and it is a "process where you become aware that everything is energy manifested."
Sanskrit: Om srija iva rtaya swaha
Japanese: On chirichi iba rotaya sowaka
9.)ZEN: represents enlightenment, and it "connects you with the I AM."
Sanskrit: Om ah ra pa cha na dhi
Japanese: On a ra ba sha n sowaka
And here are links to the mudras and the sources where I got this info:
2.) "Advanced Kuji In" by Francois Lepine. www.kujiin.com sells his books, but I believe most of them are on Scribd.
3.) "Qi Gong and Kuji-In" also by Francois Lepine. He seems to be the most popular author on the subject.
YouTube also has several playlists of Kuji-in related videos.