Cutaways description is correct. I would add one thing. The second bellcrank is mounted exactly half the distance between the bolt and the line connection. All three lines will have equal tension. It is an example of the kids on a teeter totter balance that we all did in grade school. With a single bellcrank both lines have equal tension and the full reaction force is on the pivot bolt. NOw consider adding another bell crank on the first one. Only in this case the new bell crank will have its center bolt exactly half the distance out one arm of the first bellcrank. There will be two line attached to this second bellcrank. The first bell crank is still ballanced because we have the force from two lines but the moment arm is only half. This is how a Brodak bellcrank is configured if you know how to look for it. In practice the first bellcrank is hooked to the throttle and the second one mounted to it goes to the elevator. There will be equal tension in all the lines. Working the elevator does not affect the throttle. Working the throttle will actually have a minor effect on the distance from the handle to the model. You will now rightly conclude that there is going to be some "monkey motion" at the handle to make this all simple and that is what is inside the three-line handle Brodak sells. For this system to work there must be tension in the lines. If they are slack you get no throttle or elevator response. This can be an issue when you are taxying the model. In this case the electronic systems mentioned by Steve are a real advantage. All my old carrier event models used three-line systems and I have one scale model on three-line. All the others are electronic, but I use JR radios in the Direct Servo Control mode. Another reason for not using the three-line handle for a two line model is that the three-line handle does not have any adjustment to neutralize the elevator.

EF just noticed your last post while I was slowly typing my response. The up or inverted version really doesn't matter and it relates to how the bellcrank is mounted in the model, not you engine mounting. One or the other may be easier to install in your model. I set mine so that you get full throttle when both elevation lines are pulled. With the upright bellcrank this causes the throttle link to move to the rear. This is backwards from most engine throttle arms, so you have to have a reversing bellcrank in the system someplace. Setting up a throttle linkage that moves in the proper direction and has low friction is a challenge. I have never worried about which leadout was the up vs. down line--however the really serious stunt pilots seem to think it matters. Yes a reliable taxi is possible (required in F4B competition) but see my comment about line tension above. Please ask all the questions you want and if our responses don't make sense ask us to explain again.
Chuck