Yin and Yang Skinny Parts as Loop-Backs
The skinny parts of Yin and Yang are probably referring to "loop-backs". The Up and Down pattern is evident, where "up" is a visual label meaning returning to the higher dimension and "down" means energy entering our dimension in a high-density state, whereupon it expands like the fire symbol in image F but ultimately deflates and curls up while getting "small enough" to return to the higher dimension.
Symbolism suggests to me that there is a smallness threshold, and once small enough, it loses all interaction and detectability from within our dimension, but is still there in a co-located dimension.
I've seen some alternate suggestions about Yin and Yang and considered them for a while, but now I think the skinny parts should get more attention.