I was at a physical therapist for back and knee pain a couple of weeks ago. He, among other things, performed a set of stretches that put a lot of pressure on my core (tanden). Immediately, I sense a flow of sensations emanating from my core (physically a point below navel). I asked him about it, and he said for the lack of a better word it is some form of energy released by the stretches.
The core or tanden has an important role in many eastern practices (like Kyudo, Yoga, Tai-Chi), and their philosophy (Buddism, Taoism). A Kyudo advanced student (5-6 years of practice) once told me that in occasions when he has a "good" arrow shoot, he can feel the energy coming out of tanden and flowing into his body.
Even more generally, you can see the trait in many practices based on meditation. So, if you search the word you will find all sorts of different interpretations, explanations, etc.
But the "truth" can be very simple, some form of energy is stored in the muscles of that area that one can release and control via different practices. Of course, control is achieved only through long practice, life long, but the underlying truth can still be amazingly simple. You do not need God, spirituality, philosophy, mind, love, hope, concepts, intellect ... All you need is routine practice if you want to control "it" and a good physical therapist if you want to enjoy it once in a while