This technique exerts a hallucinatory effect upon the eyesight, effectively blinding the opponent. No matter how skilled an opponent may be, they are forced to wait helpless in this world of darkness, as this technique allows the user to attack unseen. Although it negates sight, it does not stop the other senses.
In the manga and anime, this technique is called by its full name, Genjutsu: Bringer-of-Darkness Technique (幻術・黒暗行の術, Genjutsu: Kokuangyo no Jutsu, Literally meaning: Illusion Technique: Bringer-of-Darkness Technique). The databook entry for this technique drops the "Genjutsu" part.