Demonstrations of seishan
Although all Kata should be performed with a correct centre line i.e. Seichusen, Seishan is the fundamental practice of Seichusen application. Seichusen is moved in several directions with internal rotation. It's dropped vertically along with opening and closing of the body and leaning lines are used.
All in all, Seishan is a variation of and towards practical use of Naihanchi.