Questions about hitboxes, impaling weapons, rigid armor, and hit chances

[Varyon]

Originally Posted by GodBeastX

Hitting the head, even if only a single point of damage gets through, forces a HT-10 roll for Knockdown and possible stunning...

...You club someone in the face and they get -5 to the HT roll!

The rest of your post is spot on, but I should note with these two, the extra penalty for Knockdown only applies on a Major Wound (injury over 50% of HP). However, any hit that causes any degree of Shock (or would have, if the character has High Pain Threshold), causes a straight roll against HT to avoid knockdown/stunning, which is still quite useful.