Neigong is a description of a particular golden-elixir fruition—strength which moves in towards the center. That process can also be called purifying jing because once there is no longer any intent in the body there is also no qi mixed with jing, so it is pure. In stillness it is sometimes called congealing jing. It is also called making the immortal embryo because the yolk (jing) is distinct from the white (qi). There are many other ways to describe it, which is part of the reason people get confused.
