Here is an excellent two part video introduction to Zazen by the Reverend Issho Fujita.
For those of you who don't know of him, Rev. Fujita was a practitoner at Antaiji before moving to Massachusetts to help guide Antaiji's satellite, the Valley Zendo(he moved back to Japan a few years ago). The Valley Zendo website has the notebooks of Rev. Fujita and in my opinion should be deemed required reading for all of those who practice Shikantaza. Rev. Fujita has explored in his writings the physical aspects of sitting in a very unique way. He is always trying out new forms of bodywork to expand and deepen his understanding of Zazen.
Here is an interview where he discusses this and an article of his on the differences between Zazen and meditation.