I Ching, Yijing or Zhou Yi
"Oracle of the moon": © 2000 LiSe
The first trigram is the potential of the context. Usually it is the situation, but in a wide sense, it can often be the 'time', the ambiance, the actors, your abilities.
In China, history is rather written according to context that to a linear sequence of time. But we all live mentally more in the context than in the physical moment between before and after. When we remember something, we remember in the first place the 'feeling' of that event, and the time we have to guess.
See Paul R. Goldin
"The Theme of the Primacy of the Situation in Classical Chinese Philosophy and Rhetoric"
available at https://www.academia.edu/
in which he quotes two different answers of Confucius to the same question.
The second trigram is what this potential means for you and your actions. Hence the red dots and crosses in the secnd trigram. It is where you look for advice or for what will happen. If this action is what fits in the context, then it is the right one.
The story always moves from heaven (initiative) to earth (subject) to man (outcome). Without the heaven-line there is no intent towards any subject. Of course the subject can change, but this new situation restarts with the initiative: it stops - and then refocuses. The initiative can lose its luster because of things you have no control over, but it's your choice to work on bringing that shine back, or to let it go and find another interest.
Whether lines change or remain unchanged makes a difference. If the initiative changes but not the outcome is a different event than if the subject and outcome change but not the initiative, or only the outcome and not subject or initiative.
Important: a line is 'activated' rather than changed and the two sides work together. This is not about a fixed future or an inevitable destiny, but about the way you shape your destiny within the context of Heaven and Earth. The middle line is most of the three lines 'up to you'. But it does not mean that Heaven and Earth - the initiative and the subject - cannot be changed by you or anyone else.
When a trigram changes, there are other changes which add their own meaning, even though they are not clearly visible. Like music with overtones. In Kun the middle line that changes from yin to yang brings Kan into it, water with its intuitive way. If the line is unchanging, Water still has influence, but now as subconscious forces. Maybe because it is not necessary now, or it can bring its negative side into play as a lack of water.