The three-joint theory
The classic mention of this term is "understanding the three joints" (明三节）.
The whole body is composed of three joints. Each section of the body is further divided into three joints. The wrists and fingers are exceptions. The ankles are exceptions.
The three joints are defined as two solid parts with a joint in the middle.
Here is a video on the three joints theory.
Source: https://practicalmethod.com/?p=36373