Seichusen is the imaginary line that runs through the body from top to bottom straight through Tanden. In English this is often called the centre line. Seichusen is written by three characters in Japanese (Kanji). "Sei", "Chu" and "Sen". Although Japanese is sometimes difficult to translate, in this case it can be done quite easily. Sei means correct, Chu means centre and Sen means line. Chu can also mean inside as Nukina sensei once translated the Kanji for me with "It can also mean inside". Seichusen divides your body equally in a left and right side, which gives you balance and fast, smooth movement.
Many top athletes have a very good quality Seichusen, even though they do not use the word of Seichusen. It is not the word itself 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.
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.
Although Seichusen is eventually an imaginary line, it is not just a line. Seichusen divides your body into a left and right side equally, which allows for proper balance and 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. Have a look at the video "Seichusen for alignment" if you have not already done this.
Finally you can take a look to this summary of one of my seminars on Seichusen, held September 2014.