I'm not too concerned about the mudras themselves. I'm not so sure these things are "spontaneous" as harlan suggests. Hand mudras are specifically taught as part of the esoterica of kundalini yoga, shingon buddhism, as a means of cultivating spiritual energy for the specific purposes of enlightenment.

When I see the founder of Aikido in various qigong "meditation postures", chanting the kotodama (mantras) etc. etc., I begin to wonder.... granted some of this comes directly from Shingon via Omoto-kyo (which is supposedly Shinto-based).

But what I'm actually curious about is how this knowledge is common in yoga, buddhism (and buddhist qigong), as well as ninpo. What's the link?

I know what you mean. My sensei has alluded to using those in the Yoshinkan headquarters but he doesn't really teach it especially since he said they would have to site like taht for more than an hour and our class is only 2 hours lol. I might ask him about them though, he only showed us one mudra as an exmaple of a common position, basically all the fingers are interlocked inwards except the indexes which touch each other at the tip and are pointing up.

I just recorded a Discover Channel thing on Samurai and part of it showed some kenjustu school and for a quick 3 second clip showed the hand of a guy doing mudras but it was only in the middle of his thing (don't know what to call mudra practice) and I only caught about 3 mudras, I will post them up here once I get my video stuff working.

But anyone who has trained in any of the Aikido headquarters (yoshinkan, aikikai, etc doesn't matter) know of mudras?