In learning the martial arts there are two sides to the training- that which is in plain view, and that which is hidden- not because it is “secret”, but rather that is just how the transmission is set up. The teacher to student / master to disciple model is in full effect.
The first layer of understanding is called taiden- body methods and is the easiest to understand. You watch your teacher perform a movement and you copy it. They add another piece and you add another piece- most of our learning as a society is done this way- watch and mimic.
The second layer is kuden- verbal transmission which follows the taiden. Moving here opens up that, you do this to anticipate that- verbal instruction regarding the training and various movement scenarios that can only be understood once you have the base movements down as a reference.
Shinden in the final layer, and the most mysterious since one already has to have the taiden and kuden bolted down and under their belt so to speak. This is the years of experience endured by your teacher- which is a result of the years of experience of their teacher- all the way back through the history of your school/style. The finer points that can only be experienced are transmitted that way- knowledge passing from one to another in an unspoken and unexplained manner.
To put it in cruder yet easier terms to understand?
My teacher wants to transmit to me the shinden of throwing somebody so they throw me around for a bit, experiencing the lessons first hand in the moment as they actually exist, not as they would be taught in a learning environment. When the smoke settles and I get back up the shinden has been transferred to me to understand- capturing that feeling and experience in the moment of being thrown and allowing it to mature and grow in my own training. Shinden always takes years maybe even a decade to come to full fruition.