The meter indicates the amount of power received at Diode Detector. Observe how it is affected as you unscrew the micrometer head. At first there will be little effected on the meter, some ahallow nulls and then a deep null; i.e the meter reading will sharply fall the zero or near zero. The cavity is now resonating at a frequency corresponding to that of the signal in the waveguide. At resonance even a small coupling can absorb a high proportion of the power travelling along the waveguide,so that the detector dose not receive it. This principle is used,in practice,in the absorption type of frequency meter.