Cover DR

Cover DR is a term for Damage Resistance provided by Cover (based on HP and possibly Injury Tolerance or DR) to things behind it when attacks hit and overpenetration occurs.
RAW it seems like the eyes do not provide cover DR to the brain but that would be a good house rule.
MA136's Extreme Dismemberment is related. It applies to Cutting Attack which would sever. Severing requires twice the damage to cripple. So instead of >1/2 HP for limbs and >1/3 HP for extremities, it's >HP or >2/3 HP respectively UNLESS targeting Joints. Joints shifts limbs to the threshold of extremities, and extremities to 1/5 HP which would become 2/5 HP to sever.
ED only applies to a foot or leg of a target standing or lying down, so sitting, kneeling or crawling protects against it.
*why is not entirely clear, it's possible for people to "stand" in a wide horse stance or long lunge with feet over 1 yard apart, for example, which should prevent the attack for C-range weapons.
ED only applies to a hand or foot of a target that is grappling or wielding a 2-handed weapon, so avoiding grappling and limiting to 1H weapons protects against it.
*this likely assumes a two-handed grapple on the same hit location where arms/wrists would be close together similar to holding a 2H weapon. A good house rule in that spirit is to exclude it when someone is grappling with just one hand (similar to one-handed weapons) or when using 2 hands to grapple separate targets, or separate locations on a singel target.
The "the attacker may elect to carry his attack" requires "injury exceeds that required for dismembership" so in the case of a limb, it doesn't just require HP+1, but rather, HP+2 injury.
"the effective skill of the initial attack at -1" likely includes the penalty taken for the initial hit location. 
It likely implies that you must attack a 2nd limb (not extremity) if the 1st hit was an extremity. Otherwise the 2nd hit would actually be easier (-3) if the attacker went from limb to extremity.
There is no mention of applying the hit location penalty (or any other penalty besides -1) to the 2nd roll.
"Victim gets no active defence" is quite extreme, as even though it's the "same attack" it's a separate component. Like for example, if a Dual-Weapon Attack is made, separate defences are made against both portions.
Uniquely here basic damage is NOT rerolled. An effective DR is applied to protect the new location based on three criteria:
*HP/2 if a limb, HP/3 if extremity (unclear if this is reduced for Joints, since it appears to be based on the crippling threshold). This is rounded UP.
*DR of severed part
*DR of new target
Logistically this would be a much harder move to do than implied: cutting through one wrist to slash the other could not be done with an overhead chop of a sword/axe if the forearms were vertical (perpendicular to ground) for example: it would need to come in at an angle from one side or the other.
"he may opt to cut through to the other arm" does not reflect any option to target a hand instead.
"plus 6 HP for sliced meat and bone" used in the example indicates that this is not a precision hit which avoids bone to hit the ligaments.
That would likely be represented by targeting joints, which should logically enjoy reduced cover DR.
Category:Concepts