My question is, what 'causes' an effect in the end for Allomancy? You've got Investiture being filtered through a metal, but does putting it through the metal turn the Investiture cause a Steelpush, or is it putting the Investiture through your soul that causes it? At what point do you turn Preservation's Investiture into a Steelpush, or is there no one 'point' where it happens?
Okay, imagine you've got one of those play-dough machines you can stuff with dough, then press a handle on the top to make a little snake-like tube of play-dough squirt out.
Those have appendages you can affix to the front to change the shape of the tube that comes out. The metals are the appendage that determines the shape of the power released, but only certain souls can unlock those metals and use them.