Yin is the yang-wood branch (hidden Jia, Bing, Wu), the 1st month, 3–5 am — the year's first stirring.
Yin hides Jia's wood, Bing's birth (fire) and Wu's earth — a log carrying its own spark: action birthing opportunity. Watch the Yin–Si–Shen punishment and Yin–Shen clash; Yin–Hai harmony helps. Yin opens roads; it doesn't garrison them.