Ah, well, I'm probably not the best person to explain it-- I barely even managed to form a golden core.

[but getting jin ling to explain it might not do much either.]

I suppose it's easiest to call it the manipulation of spiritual energy. Once a core is formed, one's cultivational strength will grow, but if it isn't done quickly enough then their potential will be limited... those who form one later in life can't expect to be very strong.

[he would know!]