The voltage bias can switch the output light from the same waveguide $a$ to the other waveguide $b$ when the input light is into the waveguide $a$?

I am currently studying the textbook Physics of Photonic Devices, second edition by Shun Lien Chuang. Chapter 1.4 Overview of the Book says the following:

An example of an electrically controlled directional coupler is shown in Fig. 1.15, where the coupling of two parallel waveguides is controlled by a pair of electrodes via the change of the refractive index through the electrooptical effects. The voltage bias can switch the output light from the same waveguide $a$ to the other waveguide $b$ when the input light is into the waveguide $a$.

I don’t understand this description of figure 1.15. In particular, I find the follow explanation confusing:

The voltage bias can switch the output light from the same waveguide $a$ to the other waveguide $b$ when the input light is into the waveguide $a$.

Would someone please take the time to explain this?