I recently (8 months) started Tai Chi with william C C chen, who emphasizes tai chi for fighting and isn't into any mystical kind of anything. He's probably best known for training fighters that win tournaments, both push hands and also san shou (sanda)

the point I'm making is that for master Chen, the internal power is physical, bio-mechanical and not something mystical.

I'm finding it very interesting

here's an interview that he did about 3 years ago at age 75 (though the interviewer doesn't seem to know much about tai chi)

http://quest.tv/free-tv-channels/mop...-nails-theory/

and his latest article:
http://www.williamccchen.com/DOC_15.htm