Seigan is the stance wwhen holding the sword, you point it directly at your oppenents eye`s. My source for this is from, the life giving sword by yagyu munenori, chief instructor to Tokogawa leyasu, translated by william scott wilson.
The tip of the katana, is shaped so, because it is this that contains the Samurai soul, and it is this that you present to the world, Blind your opponent with your spirit, and victory will be assuered.
well this is not an exact quote from the book but my intrerpretation. but seigan is pointing the sword directly at your oppents eye`s, in the Yagu tradition.