How does the so-called SWR, Forward pwr, Reflected pwr meter work?

(Or how gullible can you get?)

A small fraction of the line voltage is tapped off. This is V1.

A small resistor is inserted in series with the line. The voltage across it
is V2.

The resistors are proportioned such that when the meter is terminated with
50 ohms, V1 = V2.

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.

It's a sort of confidence trick which works provided a lot of assumptions
are made.

Whatever is inside the box, a little toroid or tiny fraction of wavelength
of line, they all work in exactly the same way.

By the way, they make excellent little transmitter-loading indicators and,
provided the transmitter is loaded with exactly 50 ohms, they will also
indicate the power dissipated in it. Otherwise - - - ???
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Reg, G4FGQ