This Thursday in the Zoom class, Master Chen talked about two Taoist ideas 玄关 xuan guan and 设窍 she qiao. We translated xuan guan as mysterious location, where 关 historically meant a fortress, like a strategic control point; and she qiao as an empty spot/hole/orifice.
These seem like pretty abstract ideas, so I wanted to relate these two ideas to a physical example: a torus. The hole in the middle of the torus is the she qiao. The axis of revolution (i.e. the center of the ring) is the xuan guan.
If you cut a cross section of it down the middle, in 2D it is indistinguishable from the cross section of two cylinders (or two of any complementary, intermeshed rotations). This might call to mind some other examples that Master Chen often uses, such as two water bottles or two meshed gears.
You might also notice this phenomenon in other places, such as in magnetic fields, turbulent water, hurricanes, and more!