the foundation of science is to state "i know not what that is" and then go about find ways to explain it, but first and foremost admitting i know not. that's also, the foundation of shoshin. a few years back, at one of the height of aiki war on aikiweb, i was curious about what all these IP/IS/aiki talk. i thought these guys were smoking something really good, maybe they founded the new LSD. so i went out and seek and experience it first hand. in my mind, all these things are just, as you said, good biomechanics (did i mention i also worked on my biomedical enginering degree?) one of the IP practitioner did something to me, and with all my knowledge i could not explain. so i said "i know not what that is". as an experiment, i took what these IP folks offer and worked with it for about 2 years. i felt certain changes within my body. some i could explain, many i still could not. even with the vault knowledge in sciences today, we still do not know many many things about the human body. to said that we can explain everything we do is extremely arrogant. there are many things the ancient did, we still could not explain or understand.
btw, the term jin meant, using mike sigman terminology, mind directed force. it came from the old statement: heart leads mind, mind leads qi/ki, qi/ki leads physical movement. from science point of view, you have mind and biomechanics. the mind is a vast and sophisticate computer with neural networks running throughout the body, that uses various bio, chemical, and electrical processes, at least those are the three that we know of. threre are those that we can directly control and those that are on "automatic". that is just the mind part which we knew so very little about. then the biomechanics which governs by various mechanical and chemical processes and control or not control by the mind, that we know of. we still don't understand so many of these. the amount of stuffs about our body that we don't know are stagering. if you noticed i didn't mention about qi/ki portion. that there is a gap of knowledge that we can't explain very well. because that discussion will lead us to the various questions such as what is life? what is the soul? what makes us so different even though we all have the same body parts?
there are things i can explain and things i cannot. those that i cannot, i accept the terms used by the ancient, until which times my knowledge catches up with my experience, then i will attempt to explain. to do anything else is crazy talk. then again, i wasn't sane in the first place.