Relaxation of the larger, conventional muscles is a by-product of this internal focus, not the cause of the internal focus.
Trick wrote:is that with the intent-driven movements there is an deeper focus early on in the “internal” martial arts. All the actions and interactions of “Yin&Yang” of muscles, tendons, and so on are there in Karate practice, the push and pull.
This discussion has completely avoided "breath". Like Sun Lutang said, styles are not internal or external, only breath is.
One of the most important aspects of internal principle is that the lungs and breath is kept relaxed and calm regardless what is happening, even upon releasing/issuing. When I see this very good guy, no doubt about that, I see him tense up his breath and chest when striking. If you understand internal principle and can use it is really about how you can control your relaxation and tension of body, breath and mind.
Speaking about styles become problematic. Not even Xingyi does separate internal and external the same way as in Tai Chi. Some styles like Baimei and Southern Dragon do not accept that internal an be separated from external. Now, when it comes to some Hakka arts (from where Karate comes from), this distinction becomes even more problematic. They certainly have deep internal training, but at the same time, they use heavy, forceful breathing techniques and exercises, power breathing, that goes very much against the northern internal arts. However, they can learn exceptional breath control from this practice. I've also had a Hung Gar practitioner demonstrating issuing of a perfectly relaxed strike, no effort, no wind up, no tension, no special or apparent tension of breath. A power seemingly coming out from nowhere and with really, scary power. Very similar to what good Xingyi and Tai Chi practitioners can do.
I have no idea what a Karateka could do, I have never watched something similar from a Karateka. And I have much too little knowledge of what kind of internal practice they do. But I don't believe that internal skill can come out from nowhere. A certain focus of practice based on a certain understanding of internal principles should be maintained for a very long time before being able to control breath and tension and before being able to really put it into use.
If external could grow internal skill, then internal should grow external skill. But if someone asked me to throw a good Karate punch, I would be terrible, the Karatekas would laugh at me and say that I strike like a girl. If someone told me that all music is the same, so if you practice the trumpet for a very long time you could automatically play a violin I would laugh at him. The body skill required for the instruments are very different. And so could be said of different body methods in various martial arts.