The Dangai was originally both a penal colony and a passage to the Human World. The Kōryū and the Kōtotsu are remnants of its time as a penal colony, they also act as a means of defence 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 to be 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 the 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 do not use the Dangai to travel between Soul Society and the Human 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. While traversing the Dangai, Uryū Ishida and Ichigo Kurosaki (attempting to save Uryū 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 Shihōin 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 Shinigami's Zanpakutō 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 Kaikyō Kotei. 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 single-handedly 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. 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 Dangai.
Extensive research into the physiology of the Dangai can allow an individual to gain space/time manipulation abilities, as represented by Kagerōza Inaba, the 7th Seat of the 12th Division.
The Kōtotsu, is the Dangai's 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 that makes up the Precipice World, only molded 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. The 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.
Shinigami are not supposed to be able to harm the Kōtotsu, but Shūsuke Amagai, using his Bakkōtō, and Sōsuke Aizen in his Hōgyoku fused form have been able to destroy it.
Being chased by the Kōtotsu has shown to lead to a time distortion. Ichigo trained for 10 days in the Human World, waited 7 days for the Senkaimon, and spent 5 days total afterwards, before his 3-Day Bankai training. Logically speaking, this is a total of 25 days, which is exactly (at the time) one day before Rukia Kuchiki's execution after Ichigo's training is complete. However, as luck would have it, after being chased by the Kōtotsu, Yoruichi and the Ryoka entered a twisted time-space tunnel when they escaped the Dangai, making their actual arrival to Soul Society about 7 days earlier. This is stated by Yoruichi in her thoughts on the first day of training, so this left a total of 2 days training left, and 8 days following the training that could possibly be used as an extension. It is later determined that the reason for this particular time distortion was due to Urahara's technology, which made the penalty only end up being a few days. Normally a victim would have been blown away in increments of 100 years and die, as their body would not have been able to keep up with the rapid flow of time.
In addition, when Aizen destroyed the Kōtotsu, Gin Ichimaru commented that led to the axis slipping and resulted in them arriving in Soul Society a little further off from Karakura Town.