Shihan Jan de Jong OAM 9th Dan appears to agree with Gracie and Gracie in Brazilian Jiu-jitsu: Theory and Practice when they suggest that a black belt denotes a depth of knowledge and skill that makes the yudansha (black belt holder)a fully qualified teacher. That in addition to knowing how to perform the techniques, yudansha should also know why a given technique works.
De Jong included theory grades in the dan grades of his jujutsu system. A typical question was, what are the forces involved in a tai gatame ude kujuki (body set arm breaking)? The grading candidates were required to verbally explain rather than physically demonstrate the answer.
De Jong had an intuitive understanding that forces explains the why of techniques, however, he nor the grading candidates did not have an understanding of forces. This meant that the answers to the question were often convoluted and included irrelevant details.
Forces cause the change in motion or shape of all things in the environment. A change in motion, which forces cause, refers to something starting, stopping, speeding up, slowing down, or changing direction. A change in shape, which forces cause, refers to deformation. Deformation of bodily tissues can cause pain or injury. All the techniques taught by all martial arts involve forces - forces are what makes them work.
The beauty of it all is that forces are easy for the layperson to understand and apply.
How would you go about answering the above question?
1. Identify all points of contact between the two parties. This is were forces can be applied.
2. Forces are a push or a pull. That is it! Determine at each point of contact whether the force being applied is a push or a pull.
3. Determine the direction of the push or pull.
4. Determine the relative magnitude of the push or pull.
5. Determine the objective of the combined forces - pushes and pulls. Are they designed to cause a change in motion or a change in shape?
That is how easy it is to explain every technique taught in any martial art.
With respect to #5 above, different objectives of a technique require forces to be applied in different directions and maybe with different magnitudes even though they have the same points of contact.
For example, kansetsu waza (joint techniques) are used for multiple purposes. The can be used to restrain, inflict pain, incapacitate, as a takedown or a throw. They have the same points of contact but the forces at those points of contact are applied in different directions to achieve different objectives.
Congratulations. You have just nailed your theory grading (even if your examiner is not up to speed on the concept of forces).