The Viscera (ないぞう) effect makes Sabitsuki's intestines to spill forth from her stomach. Pressing the action button will make her attempt to put them back in to no avail.
The viscera effect can be found in the Flesh Walls world by navigating through the world and reaching a room with a dead black haired girl laying on the floor. Interacting with her will grant you the effect.
Upon reaching the Flesh Walls, move west, then south, then west, then south, until you reach a path that allows you to go east. Move either west or east on this path until you will be able to turn north, and go north into the narrow path. Move north towards the door of flesh, continue going north in the next area, then turn west and go through another flesh door. Move south, go west at the intersection and go in between two flesh objects.
Move east in the new area, and turn south after you go past the three white things with open mouths. Go in between the other pair of flesh objects, and move east in the new area. When the path branches, choose the one that goes south and go through the door on the north wall. Move west and through another door. Go south, then west, then south, then west, then north until you will be able to turn west. Go west, then north, then west, then south, then north between two demon-like things.
In the new area, go around these demon-like things, and move through them to the north again. Then go through the door. Move north, and through another door. Go around the flesh block you just came out of, and interact with the dead girl lying on the north to get the Viscera effect.