The core concepts are (in no particular order):
Technique levels There are four levels of understanding and practice for every technique. Their purpose is to teach basic form, moving to rapid, rhythmic movement, then adding layers of Aiki and martial Zen understanding.
Footwork Rapid entry with irimi or tenkan, to omote or ura, are characteristic of Aikido. Yusihinkai teaches O Sensei’s footwork, involving small but rapid steps for stability and speed, combined in more advanced levels with a hip/knee movement called Moon Shadow that provides significant power and timing advantages. The footwork also includes special movements for various turns, and for weight transfer. Footwork also incorporates the specific ways in which the feet and body work together to connect strongly with the ground.
Centrepole Aikido is often described as circular or spiraled, the triangular Hanmi kamae spinning. Yuishinkai sees this as a powerful and rapid rotation around defined axes (vertical, horizontal, tilted). Turning around the axis and pelvis in this way adds tremendous speed, power and stability, and profoundly disturbs Uke. This requires correct footwork and use of the skeleton.
Yin and Yang Techniques are also accomplished with an understanding of the varying balance between Yin and Yang mind and technique as dictated by the different stages of a technique and the attacker’s reactions to your movements.
Tanden Correctly moving the Tanden (centre) horizontally, vertically and diagonally at the right times dramatically increases the power of the throw and the effect upon uke.
Using the Skeleton The bones in the feet, arms, shoulder-blades, legs, pelvis and spine move in particular ways to set up entries, to compress and control the opponent, to free the thrower up to move, and to generate power.
Softness Softness most definitely does not mean weakness or “aiki fruityness”. Softness means several things, but basically it is the release of all tension in the body, allowing Ki in, allowing the optimum integration of mind and body in performing technique and with unifying with the universe. Yuishinkai cannot be done without it.
Ki Ki training is part of the Yuishinkai syllabus. All students have to be able to do Mind in the Tanden, Relax Completely, Weight Underside, and Extend Ki under stress and movement. Students need to be able start everything with Ki is Extending, and persist with ki extension during and after every action. There is also specific Ki training within the 4 levels of technique to match mind and body to the situation each level is designed for.The Tanden Ball exercise is also vital to Yuishinkai training, it forms fully 1/3 of Maruyama Sensei’s personal daily training (the rest is 1/3 meditation, 1/3 aikido). It trains mind and body in Mushin, No Mind, which is crucial for correct Yuishinkai. Sensei also says that training correctly, i.e. consistently using all the core concepts correctly will train students in Ki.
The Art of Ukemi When uke attacks nage it is with the intention of succeeding in the attack. Full commitment is required of unified mind and body – note however, that in training with less experienced partners this full commitment is performed in a way that that partner can deal with safely. Full commitment requires all the core concepts to be operating fully; it then allows uke to instantly understand what nage is doing if/when nage responds to the attack. This instant understanding of what nage is up to in turn allows either (a) uke to counter nage’s response with a Kaeshi Waza (reversal), or, (b) uke to instantly adapt and escape injury through rolling or (c) to conform with nage and teach them technique through shaping it for them, then doing rolling. All aikido techniques can be done in a throw that is safe for for the attacker, or in a way that sets up atemi (strikes) and breaks bones. As the attacker (uke) you don’t know which is going to be done until nage “tells” you via ki and technique shape, so you must be able to instantly adapt. When performing the role of nage the same full commitment is required; this leads to technique that uke cannot counter. When you can take ukemi well, then you can throw well. All the other core concepts lead to Ukemi, it is the core martial skill.
You are Mind Itself This is the meaning of Yuishinkai. A basic understanding is that you are/can do what you think. This is easily demonstrated by correctly done ki tests. Further concepts involve understanding some of the historical esoteric Buddhist and Zen views. This is how Sensei sees the world.