Equation 2.1-6 (The association between m and k1)
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.