The best-selling manga Hunter X Hunter is famous for its complex approach to characters' powers and abilities, especially "Nen", a technique that allows manipulating one's aura, the life energy produced by every living being, for various purposes. Because of how intricated Nen rules and applications can be, fights between the strongest Nen users in Hunter X Hunter are always memorable, but it can also be challenging to understand how it all works.
Throughout Hunter X Hunter it's explained that aura constantly escapes from the body's "aura nodes", resulting in a slow leak of energy. Nen users are able to open and close their nodes at will, and to stop the aura from leaking, keeping it around their bodies, changing its flow, and focusing it for various purposes. To reach that stage, Nen users need to master the "Four Major Principles": Ten, which keeps the aura around the body; Zetsu, which stops the flow of aura completely; Ren, which allows a greater aura output than usual; and Hatsu, which is the personal expression of each user, their special "Nen ability" that has unique paranormal effects. After mastering the Major Principles, these can be combined to obtain advanced techniques, for example extending and shaping a user's aura, or focusing it all in one point of the body for devastating attacks. All of this combines to form some of Hunter X Hunter's most dangerous Nen abilities. The protagonist Gon and his friend Killua were trained in these advanced uses by Biscuit, an experienced Hunter on Greed Island.
The exact effects of Hatsu are determined by the user's aura type, which is divided into six categories: Enhancement (strengthening themselves or objects), Transmutation (changing the quality of aura to another element), Conjuration (creating constructs from aura), Emission (detaching aura from the body), Manipulation (control of both animate and inanimate things), and Specialization (unique effects that do not belong elsewhere). Specialists can often be some of Hunter X Hunter's most powerful nen users. Gon is an Enhancer, specializing in close combat, while Killua is a Transmuter, able to give his aura the qualities of electricity. The easiest way of determining the aura type is through Water Divination, a test in which a Nen user emits energy into a glass full of water with a leaf floating on top, and monitors the effects.
What makes Nen different from other energy-based systems used in mangas is that Nen auras carry the desires and emotions of the users, which allows for incredible versatility. Because Nen is an expression of willpower, self-imposed restrictions or stipulations that could endanger the user can increase the power of their Nen abilities, as taking extra risks confirms the user's resolve. These restrictions can take the form of simple conditions, activation requirements, or the most powerful and dangerous ones, vows, that carry heavy consequences if broken. For example, Kurapika's powerful Chain Jail ability can be used only against members of the Phantom Troupe, otherwise, he will die.
The Nen system is incredibly more complex than what is explained here. Hunter X Hunter's author, Yoshihiro Togashi, has a real talent for crafting intricate systems of rules, which he loves to use to challenge his characters in the most desperate situations. Thanks to Nen's unique characteristics, fights in Hunter X Hunter always involve a lot more than simple physical confrontation: they are battles of the mind and require the participants to discover and understand what the opponent's limitations are in order to win. With Hunter X Hunter scheduled to return to serialization soon after a four-year hiatus, fans should freshen up their knowledge of Nen before diving again into Togashi's amazing world.