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!