There are six types of aura, and the aura of every individual is aligned with a specific category. Upon learning their affinity, one can set about learning to apply Nen in a unique way that suits their personality, which can develop into a unique skill. These six types are:
Enhancement
Emission
Manipulation
Transmutation
Conjuration
Specialization
The most popular way (and the only sure-fire way seen in the series) of determining one's aura type is through Water Divination. The test requires a leaf floating in a glass of water. It consists of a student placing their hands around the glass and performing Ren. The effect of one's Ren on the water or glass determines the student's affinity.
If the volume of the water changes, the user is an Enhancer.
If the color of the water changes, the user is an Emitter.
If the leaf moves on the water's surface, the user is a Manipulator.
If the taste of the water changes, the user is a Transmuter.
If impurities appear in the water, the user is a Conjurer.
If a completely different change appears, the user is a Specialist.
A person's aura only belongs to one of these six categories and thus it is better to base a Nen ability on the category their aura is akin to. Using abilities not of your primary aura type makes you generally less proficient at using them. By creating an ability that one isn't suited for, one risks stagnating their potential through by overloading their capacity for using Nen--something Hisoka calls "Memory Overload."
Hisoka also claims that he is able to judge a person's Nen type based on their personality (so according to him, all people of one Nen type are generally of the same personality type). However, he states that this method is similar to trying to find someone's star signs or blood type just from personality analysis--it's not always correct. According to his theory,
Enhancers are determined and simple.
Emitters are impatient and not detail-oriented.
Manipulators are logical people who advance at their own pace.
Transmuters are whimsical liars.
Conjurers are typically high-strung.
Specialists are independent and charismatic.
What affects Nen
Although the production of aura is unconscious and constant by all living beings, it is not only life energy. Aura carries with it the desires and emotions of the one who deploys it, which is what allows for Nen to have incredible versatility to those who develop their skill at using it, and also heavily influenced by mental condition and emotional state. A basic application of this phenomenon is that one can channel their aggression or malice into their aura and deploy it towards another person (i.e. bloodlust). The other person will then be able to feel that bloodlust as if it were physically palpable, and if unable to keep it from their own body by deploying their own Nen, may be physically harmed by it.
Nen also can make the physical attack or the object connected to more strength than it was. Changing the efficiency of the material so a piece of wood would be like a strong iron, it can even give it more energy to make it move for along period of time. This effect depend on the size of the object, the quantity and the nature of aura attached to.
Some ability and skills can cause other special effects that its user maybe the only one who can master it.
Conscious strengthening of Nen through pre-emptive Limitations
Nen responds to the goals and strengths of the desire of its user. As a result, a user can make a commitment that results in an increase in their abilities. If a user, for example, modifies their own body in such a way that an individual skill seems more imposing when used, their Nen ability will become correspondingly more powerful (e.g. Franklin cutting off his fingertips to make his emission ability stronger).
It is also possible to increase the overall strength of an individual skill by stating an oath that imposes even more conditions on it. For example, if a user consciously decides something along the lines of "I will only use this skill on Thursdays" or "I will only use this skill against short people" and manage to abide by the rule, that particular skill will become stronger. These conditions are called "Contracts", which can increase the raw power of an ability. Theoretically, the more restrictive the contract, the greater the enhancement in power. It is also implied that limitations that carry great meaning or are tied to emotional states also bring about a greater benefit. Furthermore, contracts which contain some sort of punishment (i.e. "I will die if I break this rule") will strengthen the ability more.
An example of this is the character Kurapika, who swore by death not to use one ability of his 'Chain Jail' on anyone but the Spiders. It should be added that even though a contract adds further limitations to using one's Nen ability, the oath/s are not the primary conditions for the ability to be activated and acts only to enhance the power of the ability.
The Unknown
Though Nen is influenced by one's mental condition and emotional state, it is difficult to judge exactly how these factors effect Nen. In certain instances, these factors may even lead to one exceeding the 100% limit cap. However, one using his powers beyond their capacity will ultimately cause strain. Potentially, this could become either a weakness or fatal to the user.
Nen also doesn't necessarily disappear following death. In fact, sometimes death can reinforce an ability's strength. If someone dies holding a deep grudge, his Nen will remain and seek out the object of his hatred.
