This may seem retardedly simple, but trust me, it's not.
Give your student the following scenario (assume C172, but modify for whatever aircraft):
You are trimmed out, flying straight and level at 5000ft at a constant, stable speed of 105KIAS with a power setting of 2400RPM.
Now ask the student what happens when he does nothing other than pull the power back to 1900RPM.
Ask the student to start by explaining what happens in terms of the 4 basic forces acting upon the aircraft. What happens, in exactly what order, why? Don't let the student confuse power with thrust. If the student can get through explaining the sequence of events in terms of the 4 forces, you can then expand into any number of related areas regarding stability and how and why it works, lift and how and why and where and in which directions it works, trim, CG, CL, excess thrust, etc, etc, etc. This discussion can be almost endless and all starting from that very simple scenario and simple change of power. I ask this on Flight Reviews and am stunned by what some folks have forgotten or never knew.
If you have a new student who doesn't know anything yet, take the same scenario, and explain in terms of the 4 forces what happens, in what order, and why.