The Shikai (始解, "Initial Release") is the second form (or first "upgraded" form) available to a Zanpakutō. To activate it, the Shinigami needs to learn the name of their Zanpakutō. This is not as easy as simply picking a name, as the living spirit of the Zanpakutō already has its own name. Therefore, the Shinigami must be able to communicate and harmonize with their Zanpakutō effectively, which requires being able to speak to the spirit within its world.
A Zanpakutō can change size and shape depending on the spiritual strength of the owner.
Achievement of the Shikai is a mark of control of a Zanpakutō, and it appears to be a requirement for advancement within the Shinigami ranks, as presumably most seated officers are capable of Shikai. It is also required in order to achieve the rank of lieutenant; as it is a requirement most captains look for. After first learning the Zanpakutō's name, the Shikai can be performed at will simply by speaking a command followed by the Zanpakutō's name. The commands vary between users and range from a simple imperative verb to a short poem. They often relate to the Zanpakutō's signature ability, or hint at the nature of its spirit. This step may be bypassed by expert Shinigami who have learned how to use their Bankai.
Constant-Release: In rare cases, a Zanpakutō, once released, will remain in that state indefinitely. This is due to the vast amount of spiritual energy the possessor has that cannot be finely controlled upon initial generation of their Zanpakutō, as such the Zanpakutō is described as a constant released form. The only known Shinigami with Full-Time Release Zanpakutō are Ichigo Kurosaki and Kenpachi Zaraki. By the nature of this type of Zanpakutō, a Shinigami with a constant release form does not need a release command nor is it necessary to even know their Zanpakutō's name. However, not knowing the name of one's Zanpakutō can put a Shinigami at a disadvantage in terms of power against those who do know their Zanpakutō's name.
Shikai Forms: Once the Shikai is activated, the Zanpakutō will change form to access whatever special abilities it is capable of. They almost always take on the form of a weapon of some kind, though the type of weapon can vary greatly among different Zanpakutō. One exception is Retsu Unohana's Shikai, which seems to take on the characteristics of an animal, rather than a weapon.
Special Abilities: Once the Shikai is activated, the Zanpakutō will change form to access whatever special abilities it is capable of. The special abilities of the Zanpakutō, like the Zanpakutō itself, have specific names that usually must be announced to fully activate the effect. As Ichigo's Zanpakutō, Zangetsu, explains, knowing the name of an attack and not knowing it makes a very drastic difference in the amount of power that attack utilizes. Some special abilities are automatically activated upon release.