Alright, I am putting myself at your collective mercy.
I have had downthrust explained to me and I have read contradictory posts.
First the explanation --
The direction the airplane travels is determined by the thrustline. So an
increase in power would cause the leading edge of the wing to pitch up
and cause the model to climb

The contradictory posts-- (paraphrased)
The direction the airplane travels is determined by the wing (and tail) and
an increase in power would cause the leading edge of the wing to pitch
down and the airplane would descend.

Now, the instances may be exaggerated for clarity, but I think that is enough
to get you guys started. I actually never use downthrust or sidethrust in my
planes. I never have learned how to use the left stick, except to push it all the
way forward!

The engine is the altitude control, and elevators are the speed control. At least that's what they taught me in full scale flight school. Not strictly true in all cases of course, but in level flight , with the airplane trimmed to a certain power setting, yes. Climb and you slow down, descend and you speed up, or decrease power and descend, or increase power and climb.I don't use sidethrust, thats part of what the rudder is for. To me anyway, all downthrust does is change the wing and tail incidences relative to the thrustline. Ken Runestrand has a good article about that in the Jan issue of RCM.

Most of the downthrust stuff was developed for craft which fly at a noticable AOA.
Ideally. the cruise speed --matches up with a zero downthrust.
All thrustline corrections are compromises . Many on models are just carry overs from th full scale practice of low powered aircraft.
On a very high power to weight ,low wing loading areobat - the need for downthrust is typically zero.

Sooo... Increasing throttle (from idle while flying) would
increase the AoA (or cause the leading edge of the wing
to pitch up) Creating more lift (and drag) at lower speeds?
And the thrustline would determine the direction of flight?

Yes and no. The thrustline has an effect on the direction of flight but it's not the factor that determines that direction. Rather it works totally in concert with the wing and tail as a trio of effects to determine how the model flies as a unit. The angles all come together to set up more of an average direction. But the final angle is more determined by airspeed.

Here's how it works as I understand it.....

Most aircraft fly with a reserve of pitch stability. This applies to most of our sport models and certainly to model trainers and ALLl full size aircraft (with the possible exception of full sized aerobatic full sized aircraft). This pitch stability tries to make the model fly at a constant or TRIMMED airspeed.

Take a pure glider for example. With a pitch stable glider if the nose drops the speed increases and the pitch stability reacts by lifting the nose until stable flight is once again established. Similarly if the nose rises speed is lost and the pitch stability allows the nose to come down and the glider speeds up again to stable flight.
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Now lets take a pure situation and add an engine to make it a powered model but with no downthrust. Assume the elevator is trimmed to where the model is flying level at 1/2 throttle in a stable manner at a constant speed. All forces are balanced and the model is happy with a stable speed.

OUTSIDE upsets on such a model will make it act like the glider above. The model will upset and then return to level flight and stable airspeed.

Now throttle the engine up WITHOUT TOUCHING THE ELEVATOR TRIM and the speed increases initially. This TRANSIENT increase results in the pitch stability reacting and lifting the nose. The nose is up and the model climbs at an angle that brings the speed back very close to the stable speed the model is trimmed for. (Side note-If the engine is powerful enough the model will continue to raise it's nose right around into a loop in it's quest for that stable speed)

Alternately if you are stable at 1/2 throttle again and this time you throttle back to idle the nose will drop into a glide with a descent angle that let's the model return to the same trimmed airspeed. The glide angle may be shallow or steep based on how aerodynamically "dirty" the model is.

Now the intersting thing to note with the above setting is that the model will always try to come back to the TRIMMED AIRSPEED. And that trimmed airspeed requires that the airfoil flies at the SAME ANGLE OF ATTACK whether it's level, climbing or diving. So for this experiment of our pure model the WING AoA determines the flight path.
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Now if you change the experiment and allow trim lever changes to force the model to fly level under each throttle setting things get interesting.....

Take our stable 1/2 throttle cruise and add power again. Now click in some down trim until the model is flying level but with increased airspeed. The angle of attack of the wing will be low, say +0.5* for argument. The model will be flying fast and with it's nose down. The thrust line angle will be +0.5* same as the wing since there's no downthrust.

Now throttle back to just above idle and click in some up trim until the model is flying level. The nose will be high and the airspeed very low. The wing's AoA will be high, probablly quite close to the stall angle. Let's say +7*. The engine's thrust line will also be at +7*.
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Adding downthrust to the model makes the throttle related pitch changes easier to deal with so that we don't have to constantly work the trim lever. What we are doing is using the downthrust to clamp down on the tendency of the model to pitch up with power and alternately easing up on that download when we throttle back. This will moderate the pitch angle change with throttle. BUT.... The overall attitude of the plane still changes a bit so the model follows neither the thrustline nor the wing line. Rather it changes all the angles with speed in a blending of the effects.
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Now pure aerobatic models don't follow the above very often because the pilot's CHOOSE to avoid the need for downthrust by trimming the models such that the pitch stability is very close to neutral. A truly neutral pitch stable model will not require any downthrust at all. But it WILL require constant pilot interaction to maintain level flight because it will have NO tendency to return to stable speed flight after an outside upset. Modern high zoot gliders often come close to this setting in a search for the most effecient operating point for the maximum duration and speed. They are more efficient but not much fun to fly if you like low key pilot work loads.
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I think I got most of it with this.....

Thanks, Bruce. Yeah, I think you covered it. Thinking back that
kinda brings some sense to things I didn't quite understand.

A high ZOOT? glider. I just may have more questions than answers now.