Dave (NM0S..... get ready to chime in here... I'm going to risk violating Mark Twain's advice ("keep silent and be thought a fool, or speak and remove all doubt) ... and try to explain the workings of the 4 State Tuner's SWR bridge operation.
First, D2 steals, and rectifies, a bit of the RF drive which is filtered by C3 to provide a "positive supply voltage" that is connected via R4 (1 k) to the collector circuit of Q1 (NPN) and also the emitter circuit of Q2 (PNP) .
That positive supply voltage is also applied, via R6 (1 meg), to the base of PNP transistor Q2, thus biasing it OFF, and preventing, for the moment, the red REV LED from lighting.
But if the 47 ohm resistive bridge (R1, R2, R3) is unbalanced because an impedance other than 47 ohms resistive is presented through the tuner elements, then an RF voltage will be developed across the C4 - D1 combo. D1 is oriented so as to rectify the RF and develop a negative DC voltage across C4, which acts as the filter capacitor.
That negative voltage is applied via R5 (150 k) to the base of Q2, counteracting the positive bias voltage applied through R6. If the bridge unbalance is large enough, the negative bias will overcome the positive, and Q2 will begin to conduct, causing the red REV LED to illuminate. D3 serves to produce a "dead band" so that the REV LED can be seen to extinguish clearly when a match is accomplished.
The greater the load mismatch, thus the greater the bridge unbalance, the greater will be the negative bias applied to Q2 and the brighter will be the REV LED.
While all this is going on the base of Q1 (NPN) is looking at the collector of Q2. As Q2 is driven to conduct, its collector voltage decreases towards ground, turning OFF Q1 and dimming (or even extinguishing) the green FWD LED.
When a matched condition is achieved and the bridge is balanced, there is no negative bias on Q2, and it is turned OFF. Q2's collector voltage then rises in the positive direction, which forward biases Q1, turn it ON and lighting the green FWD LED.
OK Dave .... did that "remove all doubt" LOL