Channel the flow of time.
The Precipice World was originally both a penal colony and a passage to the World of the Living. The Koryu (meaning "Wresting Flow") and the Kototsu (meaning "Wresting-Surge") are remnants of its time as a penal colony, they also act as means of defense against unwanted intruders. The Precipice World 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 Precipice World. The Kototsu exists to stop anyone from lingering in the Dangai and possibly exploiting this time distortion.
While the Precipice World can be used to journey to and from the Soul Society, Soul Reapers who have Spirit Butterflies do not use the Precipice World to travel between the Soul Society and the World of the Living. It is only used for that purpose by individuals who do not have aSpirit Butterfly . Indeed, even if these individuals pass through the Senkaimon with a Soul Reaper, they will be expunged into the Precipice World due to their lack of Spirit Butterflies.
Koryu (meaning "Wresting Flow") is the current that is constantly flowing through the Precipice World, 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. Attempting to traverse the Dangai, Uryu Ishida and Ichigo Kurosaki (attempting to save Uryu using his Zangetsu) were almost captured by the current. Luckily only his cape was caught, and Chad was able to break him free. Upon their escape to Soul Society, Orihime Inoue used her Santen Kesshun to block the Kōtotsu, composed of the Kōryū, and escape, but Yoruichi Shihoin berated her for this action; while the Kōryū only touched the shield area of her ability, had any one of Orihime's Shun Shun Rikka been touched by the current, the entire group would have been killed. If a Soul Reaper's Zanpakuto is used or revealed, they are also trapped by the current.
When circumstances deem it necessary, the Kōryū can be stopped using a method referred to as Kaikyo Kotei (meaning "Realm Boundary Fixation"). Normally, lower-echelon Soul Reapers enter the dimension in great numbers and using a special method, pour in their spiritual pressure 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 Precipice World's wall and as long as the user remains connected to these items for the duration. Isshin Kurosaki states that someone with his level of reiatsu can maintain this state for no longer than 2000 hours, according to the time frame within the confines of the Precipice World.
The Kototsu (meaning "Wresting-Surge"), also known as the Cleaner, is the Precipice World 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 Kototsu. If a being is caught by the Kototsu, they'll be thrown out to a greatly different point in time from when they entered the Precipice World.