Howdy-hi!
For the past two-ish weeks or so, I've been practicing a set of the Muscle/Tendon changing, shown in this video
http://www.youtube.com/watch?v=310bN783YIE
It goes through 22 different postures which bare a striking similarity to the Da Mo Fist and Palm set postures as outlined in Yang's Muscle/Tendon changing book; but which honestly don't seem to have much similarity to the traditional Yi Jin Jing set seen here
So, the postures in the video are slightly different to the ones in the book, but also so are the instructions. The video, claiming the postures to be "essentially isometrics", instructs the viewer to tense on exhalation the entire physical body (feet, knees, hips, buttocks, abs, back, chest, shoulders, arms, neck, head - the works, man!) while keeping the mind focused on a particular qi spot.
Whereas in Yang's instruction for the postures in the book, he instructs to instead "mentally tighten/push" just the fists or palms during each exhalation.
So - my question is, considering these two slightly differing sets and instructions: what parts of my body should I be tensing during the exhalations, how should I be tensing (what does he mean by "mentally tighten" as opposed to say, physically tighten...?) and where should my Yi be focusing?
Cheers!