As everything is always moving and changing, things with yin qualities are constantly becoming more yang, while yang becomes more yin. This is the basic Chinese understanding of every cycle of nature: the sun shines out (yang), then sets (yin); the moon waxes (yang) and wanes (yin); the summer changes to autumn and winter changes to spring.
The Yijing, Book of Change, works in the same way: the two kinds of line can change into one another. So when you consult the Yi, your answer begins as a singleprimary hexagram, capturing the essence of what’s involved – but you might also ‘catch’ any one of its six lines in the moment of changing into its opposite.