Nen is a technique that allows a living being to use and manipulate their own life energy. Since HXH sometimes use the word aura and Dbz had a-lot of aura in it let's just use the word aura for the time being. Aura is the life energy produced by all living bodies vital for survival. Aura from all parts of the body has a tendency to flow together, producing one mass of energy. This happens without the individual's awareness, typically resulting in a slow leak of aura continually escaping the body. If one were to lose their entire aura, it would be equivalent to using up all the energy that is keeping them alive, which would be fatal. The pores or points on the body from which aura flows out from are called nods. Controlling those nodes is the first step to being a user of Nen.
A student learning Nen trains to manually open and close their aura nodes so that they can control the flow of their aura. One typically learns this process slowly and gradually through meditation. The other method is by having one receiving an influx of aura from an experienced user that forces these nodes to open, despite how quickly it works. If I'm correct then the method is frowned upon due to its danger to the student if the user is inexperienced or has bad intents. Initiation via physically attacking another with the use of an enhanced aura typically causes permanent disability or death. Similar to Dragonball I think. Over stressing your body with Ki(Kaioken,USSJ, and just powering up beyond limits) stress your body out and can badly hurt you. Maybe even kill you. Like when Goku used Kaioken x3 and Kaioken x all Yajoribe did was slap Goku and Goku cried.
There is, however, a way to Initiate that is less harmful to the student's body depending on gently jolting open one's nodes
forces the new user to learn to control their aura flow, else risk suffering severe exhaustion or death. Once one has opened their aura nodes, he will be able to see his own aura as a shroud surrounding the body due to his nodes in his eyes being opened. Aura appears to look similar to steam from a kettle. I don't think this fits except for the beginning with making the Nen less harmful. I compare this to Full power SSJ or Master SSJ. Learning to control your state to make it less stressful to the body to the point it's like you're next using it no more.
It is possible to feel the presence of aura even without being aware of its existence. I think this could be applied to Videl when first learn Ki had Akira explained it more. I wonder why he didn't even to this day. Gohan teaching videl would have been a great explanation time for it. Since every living being emits aura subconsciously, learning to sense aura is a useful skill for those tracking living things or hunting non-living things infused with aura. An experienced user of Nen can judge the location and relative strength of his opponents through reading the output of their aura. You just have to watch Dbz to know what I'm going to say. Ki sensing.
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. This can be implied for Gohan's rage moments. This can also explain why Vegeta rage moments never helped him till he had the desire to do something about it. Hence why he got stronger then SSJ3 Goku for a short amount of time when Bills slapped Bulma. A basic application of this phenomenon is that one can channel their aggression or malice into their aura and deploy it towards another person(Blood-thirsty). Gohan when he fought Cell and Cell Jrs. That fits perfectly. I think HXH can explain Ki for Dbz guys
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