The Dangai was originally both a penal colony and a passage to the Human World. The Koryu (Wresting Flow) and Kototsu (Wresting Surge) are remnants of its time as a penal colony, they also act as means of defense against unwanted intruders. The Dangai is an isolated space which is surrounded by numerous stacked-up layers of time's current, resulting in it being a place where the density of time within it is extremely high when compared to other exterior dimensions. To be precise it is over 2,000 times denser, meaning that when one hour passes in outside worlds, an equivalent of 2,000 hours pass inside the Dangai. The Kōtotsu exists to stop anyone from lingering in the Dangai and possibly exploiting this time distortion. While the Dangai can be used to journey to and from Soul Society, Shinigami who have Jigokuchō (Hell Butterfly) do not use the Dangai to travel between Soul Society and the Living World. It is only used for that purpose by individuals who do not have a Jigokuchō. Indeed, even if these individuals pass through the Senkaimon with a Shinigami, they will be expunged into the Dangai due to their lack of Jigokuchō.
Kōryū is the current that is constantly flowing through the Dangai, in order to prevent enemies such as Hollows from using it. The current stops souls from moving, and even one foot caught by it makes escape almost impossible. It encompasses the walls, capturing anyone who makes contact with them. When circumstances deem it necessary, the Kōryū can be stopped using a method referred to as Kaikyō Kotei (Realm Boundary Fixation). Normally, lower-echelon Shinigami enter the dimension in great numbers and using a special method, pour in their reiatsu to fixate the current. However, individuals in possession of significant quantities of reiatsu can themselves suppress the current for extended periods of time but only after the insertion of four small forked blades into various locations along the Dangai's wall and as long as the user remains connected to these items for the duration.
The Kōtotsu, is the Dangai equivalent to a street sweeper. The Kōtotsu appears once every seven days to cleanse the area - killing anyone and everything that makes contact with it - to assure that no one can loiter and take advantage of the time distortion. It looks like the same material making up the Precipice World, only moulded into the shape of a bullet train, and a golden eye at the upper-center of its body. If the Kōtotsu touches a Zanpakutō or other spiritual weapon, the user and weapon are trapped, since the Kōtotsu's body is flowing with the Kōryū current. Soul Society has no means of sealing the Kōtotsu. If a being is caught by the Kōtotsu, they'll be thrown out to a greatly different point in time from when they entered the Precipice World.
