In some shonen anime, there is this trope of a hidden power system that is sometimes foreshadowed throughout the series, and eventually learned by the protagonist(s). Typically, it's a power used by essentially all characters past a certain power-level threshold, and is vital for the protagonist(s) to learn to continue fighting tougher opponents.
Specifically, however, these powers are capable of being learned by any person and have very consistent rules for all users. But, at the same time, are typically unknown to the public due to secrecy by users, an extreme difficulty curve, etc.
Usually they are used alongside the unique, natural talents/powers the protagonists were born with/trained up until that point in the story.
The most obvious example is Haki from One Piece. The second most obvious would probably be Nen from Hunter X Hunter. A smaller example would be the "Incarnate System" from Accel World.
Is there a name to describe this trope of these hidden/revised power systems?