I've always understood the working limb to be the portion of the limb that bends, end of fade to nock.
The proportion of the total work done is less in the deflexed part of a limb than in the reflexed sectioned, reducing early string tension and concentrating the work done in the outer limb.
So less deflex means that the total work done by the limb is more evenly apportioned across the limb as a whole.
Just my understanding, I could be wrong!