The enforcing and canceling effects for incident and reflected waves are opposite.
In particular, according to the directions chosen in Figure 3.27, incident power propagates on the right side and is canceled on the left side (Figure 3.27(a)), while reflected power propagates on the left side and is canceled on the right side (Figure 3.27(b)).
Therefore, directional couplers allow separate measurement of incident and reflected power by means of power meters applied, respectively, on the right and on the left side of the secondary waveguide.