that is because there is no internal/external. These are terms first coined in an article by Sun Lu-Tang, in which he catagorized Ying Yi Kuen, Bot Gua Jeurng, and TaiGik Kuen (XY,BG,TJQ) as being of the internal school and Shaolin and others being of the External School. Now these terms are thrown around by what I refer to as Internal MA Snobs, as a Holier than Thou term. ALL MA is internal.
It combines not only "core" or integrating the torso, but unifying the entire body-"From the ground, through the legs, governed by the waist, and expressed through the hands"coupled with relaxation of antagonistic muscle groups in order to most effectively recruit the protagonistic groups, and generate power. Reaxation is not flacid and limp ("we use only the ch'i to strike"-If your body moves, muscles are being used. period.) It simply means you are not "driving with the parking brake on."

Many of the isolated movements and "Ch'i-gung" method of performing certain sets, be it SLT or Sam Bo Gin, in my opinion (based on my thus far limited experience) do more to allow the student to develop the awareness of how his muscles and sketetal alignment are used, as well as breath and relaxation, allowing for the above to occur.

I have seen my teacher, with a very small movement, project a six-foot,four,350 lb high ranking BB backwards off his feet. I have felt his shock power that for lack of a better term, felt like my body was jolted. No, it's not some mysterious power, it's the years he put in training the above principles. Something few people in this day and age are willing to do.(which is why alot of "real, high level,Gung-Fu" will probably never be seen again-but that's for another thread altogether)

BennyVT's discription of ch'i is excellent, but that being said, I must admit, there are still things that defy description and boggle the mind. So for now, I guess the term,Ch'i still stands. Again, remember that Chinese Martial Scholars have terms and concepts that in a word, describe what can fill up volumes. Take the word, Sung. People write endlessly on this. Why? Because it is nearly is impossible to describe what can only be felt and understood within one's own body. This is why true Gung-Fu can only be taught through direct, transmission, hands-on, from Sifu to student. The word, Ch'i is a term that encompasses so much, from "simple" body mechanics, to the regulating of the body's internal organs and function.

I believe that it is a term used by "people who know," that rather than spending all the time explaining every sublte nuance, simply have found a term that expresses it in a word, and it is immediatley understood.
There are many things which can only be experienced and not conveyed through words to those who have not. The feeling of the adreneline response, or total fear, or the rush of skydiving,or an orgasm,or the taste of chocolate,or the birth of your own child, or losing a loved one,etc.
So....."ch'i."