The “NOT” moving

 In Classical Tai Chi, the "NOT" moving at the junction of Yin and Yang represents the essential principle of Internal Discipline. The "junction" is a specific area in the torso or joints—such as the waist or upper-quarter body—where the stationary "Yin" (supporting structure) connects with the active "Yang" (moving part). 

This concept can be understood through the following key points:

• Fast Relaxation/Energization: The "not moving" (stillness) at this junction is not passive. It requires rapid relaxation—called Song (松)—to release tension and "energizing" to maintain the structure at the same speed or faster than the moving Yang part.
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• The Goal is Internal Power: When the junction is held still and relaxed, it allows the internal energy (Qi) to compress and flow across that point, creating a "whirlpool" or spring-like effect. This builds immense internal energy in a relaxed body, which can be instantly released.
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• Preventing "Double Weighting": It is the "pivot point" where one side of the body acts as a support, allowing the other to move freely. Your body will not be “double weighted”, aka “double heavy”. Being still at junctions will enable this, and your balance will be maintained. 
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• Distinction Between Yin and Yang: Master Stephen Hwa emphasizes that to distinguish between stillness and motion truly, the Yin part must not move at all, while the Yang moves. As Yang is "hidden" within the stillness of Yin, the contrast between them is heightened, which is key to developing Fajin (the release of power).
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• How it feels: When performed correctly, the practitioner feels a stretching sensation and intense energy flowing across the junction in an otherwise relaxed body. 

In summary, this concept means the junction should not move externally, but must remain dynamically alive and "ready" through instantaneous relaxation and structural integrity.