Reg Edwards wrote:
The meter can be switched to indicate the magnitudes of either V1+V2 or
V1-V2.

V1+V2 is calibrated to indicate Fwd.Pwr.

V1-V2 is calibrated to indicate Ref.Pwr and SWR.

The key to the functionality of such is that the addition
and subtraction involve two phasors so the phase angle
between the voltage and current is taken into account.
Assuming the meter is calibrated for Z0 and actually in
a Z0 environment, the necessary conditions for functionality
are met for all Smith Chart impedance points on a particular
SWR circle.
--
73, Cecil, W5DXP