I am bit confused with the knock down action. It seems to me it always damages as if you are unarmed and that's fine, but the question is, can you do a knockdown action with a melee weapon and not change the damage to the one of the weapon used but at least lose the primitive effect of double armor?

Also If you would use the apsis shield (faith and coin page87) for a knock down action would it cause the apsis shield damage D5+1 for damage, or would it still do the standard unarmed attack for damage of D5-3 since the weapon says that weapon says you can use it for an knockdown action and gain +20 to the roll?

If you would use a power fist for a knock down action (COULD you use a power fist for a knock down action or are the only viable ways unarmed and the apsis shield) does the damage benefit from the increased SB gained from the power fist ? It would be really powerful if it did, but not breaking the spirit of the rules I think.

Honestly I would just handle that with an on the spot ruling tbh, depending on how the player is attempting the action.

Are you shield-bashing him as the knock-down? Sure use the shield damage.

Are you pimp-slapping him with the back of the Power Fist? Yes, take the extra SB on that attack.

On the whole it should remain as it is, can't really push someone down with the tip of a sword (without just running them through and killing them) but in those situations where you think there should be a change, ask your GM (or if you're GM make a ruling).