Canadian Politics Today

Ok, I get what you're saying.   

I'm still not clear on why this might cause damage to the microwave itself, though.

 -k

That aluminum foil is full of electrons that get excited when they are exposed to the force of the magnetron that creates the wavelengths that stimulate the moisture in your food you want to heat. The arcs that are created flash back and can burn holes in the walls of your oven and burn the magnetron so it quits working not unlike when a light bulb filament finally overheats and ceases to be conductive and the lights go out.