One way to explain shen is to say that it is "how we feel space." During moments of extreme fear our shen closes in on us like plastic wrap around our bodies, in English we use the term petrified to describe this feeling. Conversely, when we are in a warm safe environment with a spectacular view in all directions our shen expands way off into the distance.
(Humans are complex creatures. Some people seek out that petrified feeling, perhaps because they treasure the release which happens when the fear finally lets go and turns to exhilaration. People with agoraphobia become petrified at even the thought of wide open spaces.)
Another way to think about shen is what we sometimes call "body image," that is, the way we feel about and perceive our bodies. Skill in martial arts involves the ability to change your own body image as well as the ability to manipulate other peoples' body image. Few people could step into the ring with Mike Tyson and not feel a twinge weakness and fragility. A great fighter, just like a great actor, can make you feel like you are "king of the world" one moment and "a cockroach under a boot" the next.
I believe that hormones have a big effect on shen. How many of us have known a slender woman who once a month asks if we think she is fat? The first time this happens we laugh and say, "Yeah your a regular hippo." The second time this happens we say, "No, honey, of course not," only to be accused of being patronizing or worse. No doubt most of us have learned, through trial and error, that a failure to respond will be received as "ignoring," and that the best response is an audible but non-verbal response:
"Ahhh," can work but risks implying agreement as in, "Ahhh, I see what you mean."
"Oohhh, can also work but might be interpreted as "Oohh, interesting," which would be patronizing again.
The correct response is, "Uuuhh?" exuding slight confusion coupled with perfect acceptance of the statement.
It should be obvious at this point that shen is affected by physical training, diet and sleep. And also that it is very hard to measure analytically.
One of my students claimed not to be aware of shen for several years until we had a conversation about tension in the center of her back. She broke into a story about how when she was a little kid there was a certain place in the hallway of her house that she would pass through quickly because she always got this feeling that someone (or something) was going to stab her in the back. She overcame the fear by permanantly holding a little bit of tension in that one spot on her back. She relayed that her sister felt and did the same.
Chinese cosmology asserts that all shen experiences have a physical body component and all physical body experiences have a shen component. They are inseparable.