Both green rectangles are the same bias y·b, drawn two ways (part one's equal-area views). Pull dy/dt out of y's tip and db/dt out of b's tip. Only the part of each velocity along the other arrow counts: it drives the gold strip (dy·b) and blue strip (db·y). Pull straight across the other arrow and the term vanishes; pull backward, away from it, and the term goes negative, the overlap shrinking (the red dashed strip is cut from inside). The corner (dy·db) appears only when both move: grey when the two velocities agree in sign, red when they disagree, and either way it shrinks far faster than the strips as you pull in.