"If you see your allies losing, that's your chance. Don't step between them. Take the enemy down from behind. And if the enemy is so strong you can't even do that, then just watch your ally be killed. That's what it means to be Onmitsukidō." - Soifon
The Onmitsukidō is a special strike unit under the command of the Central 46 Chambers. It is divided into five units, each of which has its own special duties to carry out. These duties range from relaying information to patrol, discipline and assassination. If the Gotei 13 are the considered as an exterior guard, then the Onmitsukidō is an interior, covert guard. Originally a separate branch of Soul Society's army, the Onmitsukidō was traditionally tied to the Shihōin family. When the Commander-in-Chief would gain a title within the Gotei 13, the Onmitsukidō would become attached to that division as well. The Special Forces is generally commanded by a member of the noble family Shihōin, though the last member to hold this position was Yoruichi Shihōin before she departed Soul Society around 100 years before the main story line. Since that time, the Special Forces seem to be highly merged with the Gotei 13's 2nd Division.
The head of the Onmitsukidō is called Commander-in-chief (総司令官, sōshireikan). The leaders of each of the divisions are ranked as Corps Commander (軍団長, gundanchō). Given that the Onmitsukidō is currently tied with the Gotei 13's 2nd Division, each Corps Commander also holds a Seated Officer position in said division. The Commander-in-chief of the Special Forces generally also commands top division of the Special Forces, which is the Executive Militia and it is known that the 3rd Seated Officer is the Corps Commander of the Detention Unit. It may be said that the order of the five divisions also goes in order of the top five officers of the 2nd Division. (Unlike the Gotei 13 Captains, the Commander-in-chief has several bodyguards, though its current Commander-in-chief Soifon has never been seen with any).
As the 22nd head of the Shihōin family, one of the four noble families, Yoruichi was the first female to hold that position as well as being the first female to be the head of the Onmitsukidō. Because she was Commander-in-Chief of the Onmitsukidō prior to gaining the position of Captain of the 2nd Division, she was also the one responsible for having brought together the 2nd Division and the Onmitsukidō.
There are five divisions within the Onmitsukidō, for which the first has the most authority, and the fifth has the least. The five divisions in order of authority are:
Executive Militia (刑軍, keigun; Punishment Force): The first branch of the Onmitsukidō. It is ranked the highest of the five branches in the division. Their responsibilities including punishment and assassination of ex-comrades who break the law and acting as scouts during battle against Hollows.Its members are dressed all in black and are known for their cold-bloodedness. They specialize in combat using Hakuda. It is normally headed by the the Commander-in-chief of the Onmitsukidō.
Soifon is the current Corps Commander for the Executive Militia.
Yoruichi was the Corps Commander for the Executive Militia 100 years prior to the main story line.
Patrol Corps (警邏隊, keiratai): Their apparent job is to patrol the Seireitei. In the event of those under house arrest escaping they also are tasked with finding and apprehending them. Externally they patrol the various lands and dimensions to combat dangerous Hollows and other enemies of Soul Society.
It was stated that prior to his promotion to Captain of the 3rd Division Amagai served in the Patrol Corps; however, it was not stated whether he was part of the 2nd Division.
Makoto served on the same patrol unit as Amagai before becoming the 3rd Seat of the 3rd Division.
Detention Unit: The Detention Unit is responsible for the imprisonment and supervision of all criminals within the Seireitei. They also have another duty known as Special Detention, which concerns those who joined the Gotei 13 but were judged to be a danger to their fellow Shinigami. They investigate, apprehend, and keep these individuals sealed away from the rest of Soul Society within the Special Underground Detention Facility, aka. the Maggot's Nest. It should be noted that those few who are kept in the Maggots' Nest have not actually committed any crime but have been identified by the laws of Soul Society as "Dangerous Elements". This unit acts somewhat similar to a prison and mental rehabilitation center.
The fourth unit's name and function are unknown.
Reversal Counter Force (裏挺隊, riteitai; Secret Remote Squad, Correctional Force): This special unit sends information within the Soul Society. Commonly seen relaying high priority orders and messages to Captains and lieutenants from the Captain-Commander and/or other Captains.
Although the general Onmitsukidō uniform appears to be the standard Shinigami uniform, the Executive Militia uniform is different from the Gotei 13. Overall, it resembles a ninja uniform. It has no visible white undergarments and the sash is dark as opposed to white. A headpiece covers the bottom part of the face and sometimes the top, but not the eyes; they wear no sandals, but long jika-tabi, and tight bands are on the legs and arms to keep the clothes from moving too much, which both are presumably for silence of movement. In the anime, members of the Executive Militia also wear their head covering on the top, not just the bottom and wear a black sash instead of a white one.
The leader of the militia wears the same uniform, except that the shoulders and back are exposed. The reason for the empty space is the eventual usage of Shunkō (瞬閧, flash cry), an advanced technique that combines hand to hand combat and kidō, causing high spiritual pressure to accumulate at the shoulders and back, blasting away the fabric at said places.
The Correctional Force uniform is radically different. The soldiers wear a white uniform, a long piece of headgear, and a backpack.