First there is External-Internal, which means that the jing and qi are mixed. Most martial arts use this method to great effectiveness. It is high quality external martial arts-- muscles, bones and tendons become thick like chocolate.
Second is Internal-External, most advanced taijiquan, xingyiquan, and baguazhang practitioners get stuck here. It means that the body is completely soft and sensitive. While power is constantly available, the yi (mind/intent) is trained to never go against the opponent's force, so that when this kind of practitioner issues power it is in the opponent's most vulnerable place (in friendly practice it is often used to throw the opponent to the ground). Unfortunately, if the opponent gives no opening there is no way to attack. Also, at the moment of attack all jin, no matter how sneaky or subtle, becomes vulnerable to a counter attack.
The third is Pure-Internal, this is very rare. All power is left in a potential state. Because there is no jin, one is not vulnerable to counter attack. To reveal this aspect of a practitioner's true nature requires completely relaxing the physical body so that jing and qi distill from one another. The body becomes like a heavy mass, like a bag of rice, Daoists call it the flesh bag. Then one must go through the four stages of qi:
- Qi must go through the gates. The most common obstacle to this is strength, either physical, psychological, or based in a world-view. After discarding strength the shoulders must be drawn inward until they unify with the dantian. The same is true for the legs; however, the most common obstacle to qi passing freely through the hip gates is too much qi stored in the dantian. Qi must be distributed upwards and released in order for it to descend.
- Qi must conform to the rules of Yin-Yang. As much qi as goes into the limbs must simultaneously go back into the torso.
- The qi must become lively, shrinking expanding and spiraling. (This is what I'm working on.)
- This one in Chinese is Hua--to transform, like ice changing into water and then steam. But George Xu prefers to translate in as melt the qi.
Personal Update: I'm going on a classical music only fast.