Hunter X Hunter - Nen

Source : http://en.wikipedia.org/wiki/Hunter_X_Hunter#Nen

Nen is the living aura produced by all living beings, and the ability to use nen is what makes Hunters, and people like the Genei Ryodan so powerful.

An individual becomes sensitive to his/her nen once their shouko (points of the body where energy flows from) are opened. Learning to control this energy, however, is no simple task. The study of nen begins with several basic techniques, as well as a range of more advanced techniques that require experience and high endurance.

The most important ability of a nen user is their 'Hatsu.' This is their special ability - the unique manifestation of their aura. Nen-users carefully create their own Hatsu ability to enhance their own strengths, resulting in abilities that uniquely represent the nen-user. Although every nen-user's ability will be different, nen-users can be split into some general 'groups.'

As powerful as nen may seem, a fundamental concept of nen is that everything must be within human boundaries. However these boundaries can be pushed to the limit through a means of a balancing act set in a contract a nen-user defines for oneself. Ergo any 'Hatsu' created has some fundamental drawback, which is usually tailored to interfere as little as possible with the needs and personality of the said creator. For example a character in the story has the ability to attach time-bombs to people's bodies. In order to do so he must meet three conditions: touch the person while mentioning his ability (albeit without revealing that the ability is his), reveal himself as the user in order to activate the countdown, and he must explain how to deactivate it (by catching him and reciting a phrase). Being a wily and elusive character he makes it difficult to be caught and thus shores up his weaknesses nicely while maximizing the utility of his ability. It is a delicate balance system that rewards a well thought out ability and the smarter and more thoroughly prepared nen-user in a battle has a distinct advantage as opposed to the out-and-out stronger nen-user.