Hello!
I am a practitioner of the martial art system of Bujinkan Budo Taijutsu. I have been trained continually for over six years, and I have extensive understanding about many concepts (I may not be able to put all of them to use, but I get closer all the time).
I would like to analyse the ryuusui seikuken and compare it to real life martial arts. First of all I want to say that I think the fundamental reasoning to it makes complete sense, and I will explain why.
So I think it would be good to start with the seikuken. If you do not know what it is (yeah right...) I will briefly explain it:
Seikuken is a technique where the user creates a sphere with the edge as far out as the arms can reach. It is a "barrier" where the user blocks every attack at. The user makes that space his or hers. This is common to every martial art, and it has a lot to do with confidence. Quite simple, I don't have much to say about it. It's learning about oneself, where one is and how one is.
Now onto what I am really excited about, ryuusui seikuken:
This takes the sphere and shrinks it down around the users skin. This is very similar to a concept we have in bujinkan where we talk about distance. We talk about the minimum distance you need to move to in order not to be hit. This distance is where if the attack is the thickness of a sheet of paper closer, it hits.
The first stage of ryuusui seikuken is reading the opponents flow, again this is common in every martial art and you get better at it with experience but also by examining every technique and learning what happens in it. To be honest not much to say about it. This is understanting what your opponent is able to do from how he/she stands and moves.
The second stage, matching your movements to the opponent. Self-explaining. It is an important thing to learn, which builds on stage one, expanding the knowledge of how the interaction between two fighters work. This is more about aquiring the experience needed to instinctively know what your opponent does. The reason it is not something useful in real fighting is the fact that this is more a way to train than a way to fight. It's learning about the relation between your and your opponents movements.
The third stage, anticipating your opponents moves and using the his/her flow against him/her. Here it comes to something usefull. This builds on the first two stages and uses the awareness about yourself and your opponent to your advantage. To do this one uses all of the senses to constantly gauge where everything is and "feeling" where your and your opponents balance is. Due to this what you do becomes very hard to quantify, since it will be different every time. It also is very minimalistic, moving little and slowly, but early. You move as soon as your opponent has decided what to do. This happens before he/she actually starts to move in that direction. Here is where most start to think this is bogus. Well I can't convince anyone by writing or talking, this is something you need to experience. I would not believe if I had not had this done to me, and done it myself. One good way of explaining it, though, is like this; you do not take the initiave, but follow your opponent. This is not the same as reacting, it is more like moving at the same time in such a way that your position improves while you opponents becomes worse.
But what does that mean, then? Well, here it becomes counter-intuitive. Most think of fighting as the strongest and fastest have the greatest advantage, but now with being able to do this, constantly "read into the future" of what your opponent will do makes both speed and streangth somewhat irrelevant. Still, those things are great to have, but they are no longer as important. You do need to be able to move as fast as your opponent does, taking into account angels and distances and where the center of both persons movements is. Mastering this makes it less important what the opponent does, since you will always move in the best possible way against it. (Have in mind we now have left the world of techniques FAR behind).
This is instinctively knowing how and when to move according to your opponent.
These are the three stages from the series, but I would say there is one more, or you could say it is an expansion of the third. That is to apply this to everything around you. Constantly being aware of everything around you, and knowing how it all effects each other. At this point it becomes irrelevant if you are fighting one or a hundred, because it will not change how you move. That is maybe the most important thing in martial arts, to have a system of movement that is the same no matter what you do. Be it walking, eating, fighting, lifting... One way to move, infinite ways to move.
So that is what ryuusui seikuken is, now onto how I think doing this in real life would be like:
First, seikuken, learning about yourself. This is focused inward, looking at your posture and how you move. Small things like what muscles in the hip, legs, back are used when you walk. Where your center of gravity is and how to move it around. How to move the whole body as one, and how to use each joint separately. A lot of thinking goes into this and you will always come back to it, no matter how good you become.
The first stage, knowing what your opponent can do. This is taking the concept from seikuken and applying it to you opponent as well as yourself. You are both humans (I assume) and as such have the same base, what can differ here is posture and how you move, being able to see weaknesses and streangths in your opponent is the thing here.
The second stage involves training with a partner. Studying techniques, how what I do have an effect on the other and vice versa. Here angels, distances and timing are the main points, but you still keep in mind the same things as in stage one. Training many techniques does not make this faster, but the more you do have a good understanding of and can do without thought the "better" stage three will be.
The third stage is difficult to practice, since you need to leave technique behind and start to move. Here movement creates techniques, it's not something you try to do, but let happen. This is why knowing many techniques makes this better, since you have more things to let happen. The most difficult thing for me in this is you need to do nothing. As soon as you try to do something your opponent will get something to use against you. Here how things are done is more important than what is done. That is the reason to always move in the same way, a natural way that you always move in. Then fighting becomes no different from walking, and walking is just walking. Now there is nothing special, nothing for someone to take advantage of, no way of knowing what you will do next, because everything is equally possible.
I can manage this to some extent in a training setting, and I am quite certain I can come close with adrenile running, but I am still very far from being able to do this all the time. As it is now I become extremely tired after an hour of training in the way I explained stage three. Mentally, not physically. In a sense you could call this the pinnacle of martial arts, because when you master this, nothing is wrong. No matter what you do it will be positive for you. I believe every martial art end up in the same place, it's just different roads there. The saying "All roads lead to Rome" is quite fitting.
How it is depicted and what it actually is may not be the same, but how do you show something that has a nature of not being anything? I think the ryuusui seikuken of HSDK is brilliant, it shows streangth and speed are not almighty. After all, martial arts were created by those weaker to beat the strong.
I have only touched on this topic, this is wast and deep, covering every thing possible. So many small things are important, and the more you learn, the more you find. This is very much the reason I love martial arts, it is old and it is always moving forward. For that reason I want more to learn. I so often see what has been done in the past neglected as bad, only because it is old. There is always something to be learned, there are always reasons for things to be as they are. Discarding that simply for the sake of innovation and renewing without taking what makes it work with you is foolish.
tl;dr The principles of ryuusui seikuken are very much in line with what martial arts are in real life, at a high level. You can find any number of points in it that in themselves are complex and can be found in many different martial arts.
Sorry for any spelling mistakes, it is quite late here and english is not my mother tongue. Feel free to ask me any questions, I love talking about martial arts! 
