The entire I Ching is concerned with the relationship between yin and yang. Yin and yang represent two aspects. In the yang aspect, there are yin features and yang features.
This passage articulates the foundational principle that yang and yin are not absolute opposites but interpenetrating aspects, each containing the seed of the other, a dialectic that governs the entire I Ching system.
, The Complete I Ching: The Definitive Translation, 1998thesis