Before I found a teacher, I was often confused by the things people would say about internal movement. What are these people talking about? How can you use the abdomen to "do the work of" the limbs? That just doesn't make sense.
I still can't do much internally, but I've gotten glimpses here and again, so I can appreciate how people move this way. I may get corrected on this, but I believe I've found an exercise that anyone can do (let's hope it's not just me) that illustrates how internal movement maybesortakinda "works".
- Stick out your forearm. It doesn't really matter how, but let's say hold it upwards.
- Relax the hand, fingers, and wrist, and let the hand flop inward.
- Notice the fingers. If your hand is relaxed, they should naturally extend or straighten.
- Now, while keeping the hand and fingers relaxed, straighten the wrist and let the hand flop outward. If you want, you can keep the wrist relaxed and use your other hand to externally move the relaxed hand.
- Now notice the fingers. If the hand is relaxed, the fingers should naturally close or clench as the wrist is moved.
- Try moving the wrist back and forth a few times. Notice that movement is produced in the fingers without any "effort" on the part of the fingers themselves (I know technically the fingers are moved by muscles in the forearm, but you know what I mean).
I believe this works because the tendons in the forearm are a fixed length. When the wrist is moved inward, slack is created in the inner tendons, while the outer tendons are pulled taunt. This causes the fingers to extend. Vice versa when the wrist is moved outward.
This isn't exactly the way internal movement "works", but it sortakindamaybe illustrates the experience
of moving internally, or at least illustrates how someone can use the abdomen to move the limbs. The actual thing is quite a bit more complicated, but as practicioners learn to manifest and maintain certain "tensions" throughout the body (most likely fascia-related), like the finger example, if they move one part of their body, other parts get "pulled along" by the tension. This example also illustrates the need to "balance" the internal tensions.
For example, a while ago I was doing my exercises, and I extended one arm to make a slight adjustment. Immediate I noticed that my other arm got pulled back, though I wasn't putting any effort into that arm. I started playing with it, and found that whenever I moved one arm, the other arm was pulled in an opposite manner. (I sorta looked like I was practicing karate punches... hmmm...) I also noticed that as I did this, there was a very real band of tension across my upper back from arm to arm. (Too bad I can't produce this phenomenon all the time...)
So, when Mike et al discuss using the center to move, they are quite literally talking about using the muscles and other internal structures within the abdomen to produce movement in the limbs. Again like the finger example, if they move their abdomen, the limbs or whatever will get pulled along. This is, I believe, why internal movement produces such unified or coordinated movement. Practitioners are making a "single" movement within their abdomen, and the rest of their body gets pulled along. "One part moves all parts", as they say.