Four illustrations left to do! This picture is for PowerFrame's Critical Hits chapter, and illustrates the "Push & Stun-Only" Critical Hit Table. This post, I'll summarise how Critical Hits work.
In PowerFrame, a Critical Hit is any Attack that beats the opponent's defence by 5 or more. Every additional 5 points increases the severity.
The basic rules allow you to deal with Criticals simply by increasing the Damage by 2 for every 5 points of Attack Margin, but there's also an optional detailed Critical Hit system that gives combat a more risky and gritty feel.
You roll for Damage as normal (generally an Ability plus the weapon's damage rating plus a roll, against the defender's Toughness, Armour and roll). Even if you deal no actual damage, you still get to check for a Critical Hit result.
You roll a single die (not open-ended), add the Damage roll's Margin (which could be negative if you rolled below the defender), and add +1 for every additional 5 points the Attack hit by. This is your Critical Hit result. Look up the total on the appropriate Critical Hit table, depending on the damage type (Cutting, Piercing, Bludgeoning, Energy, Grapple, Push, Stun-only).
A result of 2 or less has no effect. Generally a result of 3 is ineffective as well, apart from Bludgeoning attacks or hits to the head. Results of 4 to 6 tend to be minor Stun or Bleed results. 7 to 9 become more serious, leading to broken bones and severe bleeding. A result of 10 or more is usually fatal, or at least debilitating – severed limbs, pulverised bones, paralysis, massive bleeding, and so on.
So although it's possible to inflict a Critical Hit result even if you don't deal any actual damage, it will most likely be pretty minor. Unless your Attack hit by 10 or 15 points, the best you can get with a Damage Margin of 0 is a 6. You generally need a Damage Margin of 4 or more, which is a fairly serious wound, to have a chance at taking out an enemy in one hit.
Critical Hits against player-characters do happen, but more often than not you can minimise the risk using Fortune to re-roll bad results. That said, I did once play a character who was hit with an Attack Margin of 20+ and a Damage Margin of only 1 who was very neatly decapitated (Damage 1 plus Attack Bonus +3 plus a 6 on the Critical die for a total of 10). Most of the time though, Critical Hits seem to affect enemies, and it can be pretty gratifying for a PC to devastate their foes in graphic style.