Penetration

Penetration is good for three things, in order:

In melee or unarmed attacks, Penetration may be traded for multiple hits.
Each point of Penetration allows an attack to bypass 25% of one layer of armor (while still damaging the armor). A typical mecha will have two layers of armor: one Class N Armor strapped to the outside of a module, and each module is also armored.
Any Penetration that gets past armor boosts damage by 20% per point of Penetration.

Basic Penetration calculations are straightforward, but there are a large number of talents and weapon traits that modify it:

The basic Penetration value is one-fifth of the margin that the attacker beat the defender by, rounded down.
Critical Hits add one Penetration, plus a further one-fifth of the margin that the attacker beat the critical hit target by, rounded down. (Critical hit targets are usually much higher than the target for hitting your enemy, so you will usually only gain one point of Penetration from a critical hit.)
The talent Kung-Fu adds three Penetration: two to offset the penalty for attacking with a non-weapon (below), and one more point as a bonus.
Non-weapons (usually fists and feet) take a penalty of 2 to the Penetration. If this makes the Penetration negative, then this comes off of the DC of the weapon, too (to a minimum of DC 1 at the attack's scale).
Energy melee weapons add 2 to the Penetration.
Armor-piercing weapons add 2 to the Penetration.
Armor-ignoring weapons add 12 to the Penetration. (These weapons also ignore armor entirely. The 12 points of penetration may be a holdover from an earlier time when they didn't ignore armor, so each armor-ignoring attack does at least 340% of normal damage.)
Imprecise shots (any BV attack, or any SWARM, HYPER, SCATTER, BLAST, or LINE weapon, but not multiple hits in melee) subtract 2 from the Penetration.
The Anatomist talent adds one to the Penetration if it applies here.
The Hard as Nails talent reduces penetration by 2 for personal combat.
The Hull Down talent reduces penetration by 3 for walking or rolling mecha.

If you have Hull Down, you ignore the worst effects of ARMORPIERCING weapons, and most critical hits. (And even some ARMORPIERCING critical hits!)
Similarly, Hard As Nails ignores ARMORPIERCING or most critical hits, but not both.
Don't expect any critical hits with an imprecise attack, unless it's ARMORPIERCING.
Similarly, don't expect critical hits with martial arts, unless you have Kung Fu.
The Penetration-reducing talents are only useful if somebody wields an ARMORPIERCING weapon against you, or scores a critical hit. Against most hits, they do nothing.
Even the lightest suit of armor can save you from the worst of a critical hit; if you have light external armor, and heavy internal armor, the penetration will be used up against the external armor. Even at personal scale, armor will save you, because you don't want penetration to go towards multiplying the damage.

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