The Rule

Iron Heroes uses a system of 'attack challenges' where you can take a penalty to your attack in return for a small benefit. Among those are the Bonus Damage challenges, which grant +1 damage for a -2 penalty, +3 damage for a -4 penalty, or +6 damage for a -6 penalty. These rules build off of those for simplicity.

Power Attack

Requirement: Power Weapon

The bonus damage when taking attack challenges are increased to the following values when using weapons with the power descriptor:

Expanded Mastery 3:

Permanently reduce your BAB by 1 in all situations. When using power weapons you receive a +4 bonus to damage.

Expanded Mastery 4:

The bonus damage when taking attack challenges are increased by:

Expanded Mastery 5:

Requirement: Must be using a two-handed weapon or a one-handed weapon in two hands.

Permanently reduce your BAB by 1 in all situations. When using power weapons you receive a +4 bonus to damage.

Expanded Mastery 7:

Requirement: Must be using a two-handed weapon or a one-handed weapon in two hands.

The bonus damage when taking attack challenges with power weapons are increased by:

The Justification

Power Attack is a bit weird. It's both too weak and too strong in many common situations. This stems from the fact that you're losing a percentage of your (average) damage and only gaining a flat bonus. So, PA becomes worse and worse as your average damage rises, because the penalty just keeps getting bigger. What's more, every point of PA you add actually reduces the value of the previous points! This means that PA is self-capping, or that it has a maximum useful value where your average damage output is highest.

Except in degenerate cases, this maximum lies somewhere between a -0 penalty (not using PA at all) and a -6 or -7 penalty. That is, once you get your first iterative, you really won't be power attacking for any higher amount if you're paying attention to the math. The exact location of the maximum depends on the PA multiplier and your average damage (without PA). For example, if you are using the ordinary 1-1 PA multiplier for One-handed weapons, then it is easy to see that once your average damage hits 20 it actually hurts you to PA any further (each point of PA raises your damage by +1 and then lowers it by 5%, so once that 5% becomes greater than 1 it's harmful). When you factor in magic weapons, Str boosters, and iterative attacks (which act as damage multipliers), your damage quickly comes close or even surpasses the boundary point where PA becomes harmful, making it more-or-less useless.

In those 'degenerate cases', though, is where PA is broken. If you can get a very large difference between your attack bonus and the enemy's AC, then it's advantageous to PA for a lot. If your attack bonus is high enough that you only miss on a 1, then you can use PA 'for free' to some extent, as you're not actually taking a penalty. You can get a very large difference like this by somehow gaining a touch attack against a high Nat Armor foe (like dragons), or by denying Active Defense to a high-level character in Iron Heroes. In the opposite direction, PA becomes excessively useful when the enemy's AC is so high that you can only hit on a 20. In this case, you should simply Power Attack as much as you can, as larger penalties don't hurt your damage any further. Either of these cases can result in Power Attack producing upwards of 30 damage for no appreciable cost (besides the feat slot).

The upshot of combining these two things is that Power Attack should grant higher multipliers at higher levels, and you shouldn't be able to Power Attack for more than a fairly small amount. This keeps PA useful at higher levels while limiting the abuses from the degenerate cases.