I prefer and seek out simplicity and cross cultural explanations, which has been a complicated and rewarding journey. I know this is a thread specifically for Jing, but it seems Qi, Shen and Dao are important to the overall explanation of Jing and its mystery. Here are my thoughts on Jing, Qi, Shen and Dao from my limited understanding. I hope you enjoy and can take something positive from it.
Jing (chi'n) is basically power, not just physical force or brute strength or what have you, but true power (hence 'essence'), like over all health and well being. Jing is the power that fuels life, hence the referral to a battery. The Lower Dan Ti'en (tan den) in the abdomen holds the sexual fluids, which produces life, a most powerful force and is known as the Seat of Power in martial arts world wide, as it is where all strikes and movements originate from for optimal power and evasiveness and where all breath is taken in, retained and transformed into higher forms of Jing, Qi and Shen. The Jing center is related to the Svadisthanna Chakra (Wheel of the Seat of Power), related to Water- harmony, healing, strength, power, sexuality and intimacy.
Qi (ch'i) is energy, the power and constant flow of electricity and oxygen (among other subtle forces) internally (nei qi) and externally (wei qi). If Jing is the battery than Qi is the current that the battery gives off; the breath, oxygen and radiation of life. The Middle Dan Ti'en in the solar plexus retains the fire of life, or Will Power. In martial arts we strike this area to knock the breath out of the opponent, a metaphor for loss of power... once breath is gone life is gone. It is related to the Manipura Chakra (Wheel of the Brilliant House), the microcosmic Sun, the house of Will, related to Fire- passion, determination, ferocity, personal power and health (possibly the Fire of Illumination).
Shen is spirit, often referred to as mind (similar Yi [intent]), the subtlest force of creative power. If Jing is the battery, Qi the electrical current than Shen is the electrician. The Higher Dan Ti'en in the forehead is the realm of thought, intention and consciousness. It holds the most important organ of the human body, the brain as it is connected to and controls our entire nervous sytem. In martial arts when the mind is calm the body just reacts and makes the fight more like a dance (oh, Bruce Lee!), on a more advanced level we learn to project emotions through the eyes or bring up the opponents own negative emotions to confuse and over power them. It is related to the Ajna Chakra (Wheel of Command), the central station where all information comes and goes, is retained and pushed to the subconscious for future evaluation or termination (transformation, as you can never truly terminate a thought). Its element is Consciousness and is related to the five brainwaves, learning, insight, ESP and various telepathic and psychokinetic abilities.
Dao (Tao) is the empty force, stillness, static, literally the 'Way' ('do' [pronounced doh] in Japanese). It is the beginning and end of all life and existence. If Jing is the battery, Qi the electric current and Shen the electrician, then what is Dao? What is Dao? I'll leave that to the Immortals! (Not so funny) humor aside, it is the state in which we flow, act, react and 'let go' and are non-attached. It may or may not be related to the Sahasrara Chakra (Wheel of a Thousand Lotus Petals), at the tip of the head (Pa Hui). It's element is all and none, but can be comprehended as Unity or Connection and represents Divine connection, prophecy, Enlightenment. When this Chakra is fully open all other Chakras are open and flowing smoothly, creating what the Vajra Buddhists call the Rainbow Body, and others refer to as Liberation, Enlightenment, Conversation with the Holy Guardian Angel, Sainthood, etc.
All four powers, or concepts, are empowered by breathing, posture, movement, meditation, all included in nei gong and qi gong (as you already know) as well as in certain types of Yoga and various Mystery Traditions (although I would say the Daoists and Yogis [including Tibetan Yogis] have it best]. Certain Masters also say that faith and prayer are fundamental to the development of Jing, Qi, Shen and Dao, especially for Shen and Dao, as it is the Dao, and its myriad enlightened inhabitants and primordial beings, that place Realization upon the practitioner, but that is just opinion, for we still cannot fully explain the amazing power of faith, if that is even possible.
There are many complicated explanations of Jing, QI, Shen and Dao related to organs, Yin-Yang and the Five Elements Theory, but they are just words for concepts and expressions of aspects of life that must be developed and experienced, many of which are only taught to the highest disciples in the temples. We may gain health, strength and happiness, among countless other treasures, but the truest and purest understanding is only for the Realized few (by Realized I do not mean Enlightened, because I have met very powerful Daoist Wizards, and they are not Enlightened [at least not in the Buddhist term] but very powerful and knowledgeable in their craft... Realized may not be the right word, it just sounds nice in relation to the subject). Although I believe that learning from a qualified Sifu or Guru is fundamental, there is no reason not to gather, share and experiment (carefully and patiently) with the knowledge that is presented in video and written word!
I have added the Chakra correspondences because I believe it will give a greater understanding of the power of Jing, Qi, Shen and Dao, especially since it was traveling Chinese monks who frequented the temples of India and learned many things of spiritual, energetic and physical cultivation from the Great Yogis and Sadgurus (highest realization of God/Creative Source in Yoga).