Quoted from Finrod:
I don't understand its purpose, though. I would imagine that it has to do with both right flippers being on the same flipper switch, as opposed to the usual double flipper switches used on other Williams machines with upper flippers, but I don't understand enough of the electronics to understand what's going on there. It's also making fixing it a pain in the sawdust.
It is good that you are trying to understand the WHY.
Normal lower/upper flippers are wired with a cabinet double leaf switch. The first closure completes the lower flipper. The second closure completes the upper flipper. This allows the flippers to be energized independently of each other but always the lower first and upper second. You can hold (cradle) a ball on the lower flipper while using the upper flipper independently.
Simultaneously energizing the lower and upper (a hard press of the cabinet button) will draw high current from the flipper supply at the same time. This will drop the voltage and potentially affect the amount of current that can be delivered. The strength of the power stroke of the flipper is related to the amount of current that flows through the solenoid.
The wiring on Black Knight 2000 is different. The wiring on the right flipper removes the voltage supply from the upper flipper until the lower flipper completes the power stroke. When the lower flipper EOS opens and closes the second switch in the stack the second switch provides the voltage supply to the upper flipper which will then energize. This changes the current draw from parallel to serial. The lower flipper draws all the high current first and when it has completed the power stroke then the upper flipper is then energized and draws high current.
So why do this? The most likely reason is to allow most of the initial high current draw to power the lower flipper to get the ball up the ramp to the upper playfield. The ramp is steep and the flipper solenoid is FL-11630. It's the strongest flipper solenoid Williams used when the machine was manufactured. Williams later introduced two stronger flipper solenoids (FL-15411 and FL-11629).
If you want something comparable you could change the cabinet flipper button to a double leaf switch and wire the cabinet switch like every other game with lower / upper flippers. Along with the switch change you can change the lower flipper solenoid to FL-15411 or FL-11629. This will increase the power stroke of the lower flipper to get the ball up the ramp in the case when someone presses the button and activates both flippers at the same time. The drawback is that now you have a stronger flipper hitting the drop targets on the lower playfield that can increase the risk of mechanical damage from the increased power. Not to mention the potential increase in speed through the U-Turn. It is fast enough already.
So many choices ... but in the end the only thing that matters is the game flips!