Doing an "internal" art means that you have totally re-trained the way that you move so that it is instinctive (which is why the starting Taiji forms are done so slowly). If it's instinctive, you can't switch back and forth.
That's a very interesting idea, but I think I am a living example of some middle ground between the two extremes.
I know I can do "techniques" with good footwork coordinated with normal strength. However, I also know I can "inflate" a bit which to me kind of feels like I fix my entire posture, and maybe at some points it kind of feels like my extender muscles are engaged as fully as possible without actually extending my arms or making them tight. I feel the back of my neck "open up", some muscles in my chest that I don't consciously control thoroughly relax and kind of get out of the way, my knees bend, I feel the weight of my body more directly on feet even though I feel light like I can move very fast, and my wrists pretty much feel a heaviness similar to how I described the bottom of my feet. I can do techniques where I feel much more connected to the partner's center, and maintain that feeling by kind of sneaking my body closer to the partner as my arms retract and twist, and then sneaking my body away from my partner in the new "combined" direction we set as my arms continue to maintain the connection (keeps something floating in the uke between our centers) by extending and twisting - until "something" in my unified body movement catches a bit more tangibly and uke can't directly feel why they are being so compelled into the technique. I'm still working on relaxing some strange muscle (muscle group) on my sides right under my arm pit. I can feel that gets in the way lately. Regardless, while I think what I am typically doing is certainly not "highest level martial arts", I'd say it is typically beyond normal-strength as well, and I can generally switch back and forth a will when I'm teaching people and they make a noticeable change when they feel me doing what I would call the normal-strength way versus the more kokyu power approach. Would your opinion be that my description is "advanced normal-strength" or "novice kokyu power"? It's definitely not "normal" with respect to all of the experience I have had with others, and it is certainly not a strong as some of the aikido sensei's and Chen style Tai Chi teachers I've felt.
Wendy, when I visited your dojo, I found opening up to be very difficult for me to do - but I admit that I saw and felt your aikido sensei doing it inspirationally well. I would say that I basically couldn't do much of that at all given the constraints of that class - which I think did a wonderful job of creating the opportunity for me to work on being able to do such things just inside of normal striking range.
I think developing this kind of thing in more of a combat type setting really challenges you to take the idea of "fudoshin" immovable mind/body to a whole new realistic level. I see it a a very physical example to approach the kind of "complete self trust" required in high level aikido.
I think that visiting your dojo and developing my comfort level to be able to hold myself properly under that much pressure might be the experience I'm looking for to make the physical break-through of always being able to move in a way that is beyond what I would call normal strength.