Doh, looks like you just hadn't seen his reponse yet.
Sorry Mathew ^^;
I'll interject here, while Japanese doesn't have an exact term for Jin, they do often use the term "hiriki" or "elbow power" (referring to the specialilzed type of power you use when you train the body using the bow, or other weapon).
And from my experience, it's basically yes, a willfull coordination of deep muscle tendon (rather than muscle itself).
The Qi/Ki paradigm that Mike refers to (correct me if I'm wrong here Mike) is connecting that Jin/hiriki with the power developed by specific breath related exercises. (thanks by the way for the hen/ha type hint that you gave earlier, realized some more stuff today
The breath related exercises develop a type of power/skill that is used in conjunction with your already developed Jin/Hiriki skill.
I'll also bet, unless you already have structure/Jin/Hiriki skill, 10-1, doing the breath exercises won't be nearly as useful, and any skill gained from it will be kind of dicey...
It's why even the old JMA peeps used to train simple weapons until their legs gave out. They had to first develop a solid structure/hiriki before you could even begin to seriously start to develop the breath power which combined was referred to as "ki/qi"
Just my two cents
*Ohh mann... getting ready for the hot seat