Sushi is gradual because took about half a year
while Gon and Killua's way is a sudden way because they need to awake Nen within 24 hrs.
So, they have to find out a way to keep the aura in their bodies within time.
Nen is used to control the power which is called "Aura" that flows out of the bodies. This power flows
out of everyone's bodies in only very small amount.To keep the "Aura" floowing
in the bodies, you need to know the technique called "Ten."
Setsu is used to disable all Ten in your body. It is also use in body recovering from great exhaustion.
Ren is used to let the "Aura" out of the body more than usual.
Aura is the power that flows from inside to outside of human bodies.