Red = edit
The armor pre-damage multiplier allows a weapon to eventually kill anything that does not no-sell the damage type. It may take lots of ammo, but it will die.
I know for sure it damages armor when no HP damage is taken. (Tested unwillingly by plasma against me.)
I do not know if this is correct, but my hypotesis that the Impacting Damage (Before being reduced by armor plate) is multiplied by APD and applied to impacted plate.
That way, if a minigun does 30 averaga dmg, Sectopod has (guessing here) 50% kinetic and a minigun has 5% APD, a shot from minigun will do 0 to 1.5 armor damage per hit. In 10 hits, that would be 0 to 15 damage, 7.5 average. But once it starts damaging HP the armor damage will rise.
A laser minigun, with it's 50 average dmg and Sectopod having 150% Laser, assuming it also has 5% APD, should be even more deadly.
150*5%=7.5 .
Every shot doing 0 to 7.5 armor damage per shot, averaging at 3.75.