This is the Equation 2.1-6 it the paper.
The x-axis is the proportion of males in the case group (k1).
The y-axis is the coefficient describing the association between moderate effect (m).
E1 is the exposure proportion among case women. (%)
ME is the moderate effect; Moderate effect is the ratio of OR in men and OR in women.
Green line stands for the association between k1 and m (Equation 2.1-6).
Black dash line stands for the reference line (m=y).

You can control E1 and ME to understand their effect, and you can find that k approximate m in most situations.