Re: Are you invincible if you possess Aiki?
Aiki is not mystical, it is a way of training the body which makes more use of unified opposing forces. There are training methodologies which stress how to develop tissue and fascia connection to develop whole body strength. "One part moves, all parts move.". There are methods for developing coiling or spiral power by learning to rotate the joints and tissue from the central axis of tanden ( dantien in Chinese) There are methods for developing an enhanced "pump" action in hands and feet.
And so on.
Some of these methods are related to aspects of yoga and supposedly entered China via India with the monk Ta Mo. Many of these exercises were originally intended to strengthen the body for prolonged periods of meditation but quite quickly were seen to be effective in other areas, especially for monks who needed to protect themselves.
People who are practicing this way are generally not mystical but quite practical and looking for demonstrable changes in receiving and projecting power. What is interesting in this is that much of O Sensei's writings and lessons can be read as technical explanations clothed in the language of Eastern symbolism, much of it revolving around In and Yo, the Japanese version of yin and yang and how to unify these forces in oneself.
One interpretation of this, also known as Heaven, Earth, Man, is learning to stand in an immovable posture so that when moving there is an internal stability that makes it harder for an opponent to throw but also increase the power of throws.
I doubt that most of this can be successfully understood without feeling it. If you are ever in The Netherlands you are welcome to visit and get a sense of other ways of training.
Good luck with your training,