Anyone who stops by this blog knows that I'm fascinated with the mudra of zazen. I'm enthralled not only with what Dogen calls "The Buddha Mudra", but also the actual mudra of the hands. I think there is something physiologically profound about sustaining the and posture that makes zazen a real cultural innovation.
I recently ran across this fascinating discourse on the Cosmic Mudra by a priest at the Austin Zen Center: http://www.rawzen.org/?p=231
I highly suggest some of the other articles on the Raw Zen blog. He has handouts and courses on "Nueral Zen" that explore the science behind Dogen's instructions.
Here are a few more pieces to chew on: http://www.arcatazengroup.org/pdffiles/newsletterarchives/na_july2004.pdf
and of course Muho:http://antaiji.dogen-zen.de/eng/201203.shtml