Resurrección

returns the essence of an Arrancar's offensive Hollow abilities to their humanoid bodies.

Overview
Arrancar seal the nuclei of their abilities within the form of a sword, entirely different from what the Shinigami use. When they release their weapons' seals, they unleash their true power and their true form. The only time they can return to human form is when they reseal their powers in sword form. Changing their form without resealing their powers in a sword is the same as burning off an arm, and if they were to discard part of their released form, while in it, they can never return to normal again.

Most known Resurrección give the Arrancar user an animal-like appearance. While most unreleased Shinigami Zanpakutō take the form of a katana or wakizashi, there is a significantly broader range for Arrancar (such as a sai, axe, a more fictionalized weapon like Nnoitra's Zanpakutō, or a body, whole or otherwise, like Aaroniero Arruruerie's Zanpakutō, or Lilynette Gingerbuck).

Because their powers are sealed in their Zanpakutō, an Arrancar's release will usually transform their body into a Hollow-like form, allowing them to use all of their abilities freely. This form usually reflects what the Arrancar looked like as a Hollow, although to highly varying degrees. A Resurrección restores an Arrancar to its "true" form, resulting in significantly various increases in speed, strength, stamina, durability and spiritual power, as well as allowing access to both more powerful variations of previous techniques and completely new abilities. Some Arrancar even gain new weapons as a part of their released form.

Resurrección: Segunda Etapa
Ulquiorra Cifer, the 4th Espada, has demonstrated the ability to attain a second form of Resurrección called "Segunda Etapa", and to date he is the only Arrancar capable of the transformation.