The Water 7 and Enies Lobby arcs in One Piece added a lot of lore to the series, and did a lot in particular to expand on the makeup of the World Government. At this time, it becomes clear that while the Marines are the main force the World Government uses to combat pirates, they also have a variety of Cipher Pol squads that conduct more secretive missions.
CP9, a squad of assassins, are the main villains in the Enies Lobby arc due to their intention to execute Nico Robin, who initially joins them willingly. The members of CP9 each have certain abilities which are unique to them, but all of them also employ a series of techniques known as Rokushiki.
What Are The Rokushiki Techniques?
Each of the Rokushiki techniques grant their users superhuman abilities. Geppo, or Moonwalk, is a technique which involves kicking the air at superhuman speed to lift themselves higher, in what is essentially a form of flight. The technique works best using both legs alternately, but CP9's Fukurou has been seen using it with one leg being held onto. Restraining both legs renders the technique unusable. After the time-skip, Sanji masters his own versions of the technique, Sky Walk and Blue Walk, which work both in the air and underwater respectively.
Tekkai, or Iron Body, makes the user's body temporarily much heavier and much more durable to damage. Extremely strong physical attacks like Luffy's Gear Second can break through it, and the user remains vulnerable to non-physical attacks. The user is also unable to move while activating the technique, except for CP9's Jabra, who uses it in a form of kenpo. An unknown CP0 agent recently used Tekkai to interfere in Kaido and Luffy's fight on the former's behalf, weighing Luffy down and allowing him to be defeated.
Rankyaku, or Tempest Kick, involves kicking with such force that blades of pressurized air are launched at opponents. CP9's Kaku uses the technique with both his neck and tail while in his giraffe form. Rankyaku has been shown to be powerful enough to slice buildings apart with ease. Soru, or Shave, allows the user to move at extremely high speeds, similar to the earlier villain Captain Kuro. Unlike Kuro, however, Soru users are completely aware of their surroundings while using the move, and it is thus far more dangerous in battle.
Shigan, or Finger Pistol, involves pushing one or more fingers into an opponent's body at high speed, acting almost like a gunshot. That said, users risk breaking their finger on more durable opponents. Kami-e, or Paper Art, makes the user's body extremely flexible and allows for easier dodging of attacks. This ability has been used only a few times in the series, so little is known about its weaknesses. Rob Lucci alone can use a secret seventh Rokushiki technique, the Rokuogan or Six King Gun. By placing his fists together in front of his target, Lucci can launch an obliterating shockwave which blows them away.
Who Else Uses Rokushiki?
Because Rokushiki takes a great deal of power and skill to master, many users are unable to use all 6 techniques. Soru has the most known users, and seems to be the easiest technique to master. Aside from the members of CP9 (except for their leader Spandam) and CP0, a few high-ranking marine officers have been shown using one or more Rokushiki techniques. These include Luffy's early companion and friend Koby, Captain Tashigi, and Vice Admirals Dalmatian and Momonga. Who's Who, a member of Kaido's crew, is a former CP9 member and thus also uses Rokushiki. Vice Admiral Vergo, in reality a member of the Donquixote Pirates, also used several Rokushiki techniques.
It wouldn't be surprising if more high-ranking Marines are revealed to have mastered Rokushiki, and with CP0 seeking to capture Nico Robin and kill Luffy, it's fair to expect that it'll be a big part of the series to come. Rob Lucci and Kaku, the two most powerful CP9 agents, are now also members of CP0 and will likely face the Straw Hats again before the One Piece comes to a close.