The Seichusen literally means Correct Centre Line.
Having said that, the Kanji for “Chu” (Chu from Chudan or Naka from Nakadaka Ipponken) can also mean ‘inside’. Seichusen divides your body equally in a left and right side, which gives you balance and fast, smooth movement.
If you have enough knowledge, experience, skill and imagination, Seichusen can be used for a variety of things. It can be used to for example
The increase of striking force has to do with the alignment of the body.
The black line is the centre line seen from the side. The white dot is the exact centre of the body, the centre of gravity. This is also called Tanden or Seichushin. As we have to deal with an opponent who also has a Seichusen, there is a plane that connects both Seichusen of yourself and the opponent. This is called Seichumen.
Seichusen should be squeezed through Seichumen as much as possible to achieve maximum effect.
Seichusen is the imaginary line that runs through the body from top to bottom straight through Tanden. Therefore, you have to consider a radial axis as well, which is called Seichujiku. In English this is often called the centre line.
A radial axis means not just front viewing. It concerns all directions.
Many top athletes have a very good quality Seichusen, even though they do not use the word itself. It is not the word that is important, but it is to be aware of your posture, alignment and therefore centre line.
Consciousness of Seichusen can help to improve the quality of your training and finally your performance.
I had the priviledge to have many many private conversations with Yokoyama Kancho. One of the things that we talked about was Kamae. Not just because we were writing his book, but because Kamae is a key feature of Budo.
We talked about being able to see someone's level of skill based on their Kamae. It does not tell you everything, but like you can spot a young and an old person... we can spot a rookie and experienced Kamae.
Does your Kamae have Seichusen?
Dr. Hideo Takaoka founded "Shintai Ishiki" which translates into Consciousness of body. One of his books is all about Seichusen and how this is present is all physical activities. It is this consciousness that is necessary to have if you want to improve your skill.
Imagine simply an average person running or a track runner. Running involves coordinated movement of the hands and feet, which move in unism along the centre line of the body. The faster the hands move, the fasters the legs move and vice versa. As they run their head stays still and the line moves practically straight forward. A runner never bends his Seichusen. Even though a runner might lean forward slightly, the whole line leans forward. You can compare this to Junzuki no tsukkomi, where the line is leaning but is still straight.
A runner has probably never heard about Seichusen, but still has a good Seichusen. A runner might get proper instruction, but finally it is down to talent and especially hard work. If you want to improve you have to train. It is training that allows you to improve the quality of your Seichusen and your awareness i.e. Consciousness of body.
If you have enough experience, you can apply Seichusen for many things.