In virtually all the martial arts, black belt denotes the highest level of achievement. ... By the time the student attains a black belt his knowledge and skill are of the highest class. In addition, his depth of knowledge makes him a fully qualified teacher. Rather than merely knowing how to perform the moves, the black belt is expected to know why a given move works. That is, he understands the biomechanical principles that underlie the move. The principles of leverage, of body control and mechanics. This deeper knowledge makes him a far better teacher than someone who merely recounts a series of moves. Moreover, such knowledge allows him to invent new moves and combination and so develop a more personalised jiu-jitsu. (Gracie and Gracie 2001, 17) (emphasis in original)
While the expectation that black belts are fully qualified teachers that know why a technique works in addition to knowing how to perform it is laudable, it is definitely not realised in practice or the martial arts literature.
This is another unique feature of the teachings of Shihan Jan de Jong OAM 9th Dan. He uniquely attempted to operationalise Gracie and Gracie's expectations of black belts by including gradings in Ikkyu (1st Kyu; assistant instructor) and yudansha (black belt) gradings that examine the candidates knowledge of why a technique works. This included theory grades where candidates are quizzed on the theory of techniques. There are grades where candidates are required to demonstrate specified techniques and answer theory questions pertaining to those techniques and their variations; thus fulfilling both of Gracie and Gracie's requirements of a black belt. There is also teaching examinations where the candidate is assessed on, among other things, their knowledge of the theory of techniques.
The problem with these gradings, and why de Jong's goal is not realised in these gradings, is that the body of knowledge associated with why techniques work does not currently exist. My book sets out to start to fill the void that is that body of knowledge.
Having said that, it takes nothing away from de Jong's unique fledgling efforts in realising Gracie and Gracie's expectations of black belts.