yes seems good but you need to think something for champions

surely sunder multiplied by troops count is good

but what for champions? 1 armor reductions is more usless than backswing you would never use a axe on them

I dislike that this ability applies a stacking penalty - at present, the maximum cutting defense available comes from Masterwork Chain (52 defense) if you have it, or Champion Plate (37 defense) if you don't.  Plate and regular chain are equivalent, at 32 defense against cutting attacks.  This means that if I keep a couple groups of 9 axemen in my army, I can almost completely negate the defense of any unit relatively quickly, even if they have the best armor available.

Moreover, since the value of any kind of Plate armor is doubled against blunt attacks, how would this affect the armor's performance versus blunt weapons? Does the penalty apply only to the base value? Does it apply to the value of the armor vs cutting and continue until the value vs cutting is zero? Does it apply to -1 to base defense and -1 to defense versus whatever the armor is good against?

How does this apply to trained units? If I have a group of 9 axemen attacking attacking a group of 6 spearmen equipped with leather (no shield, so total 8 defense) is the affect that each spearman loses 1.5 armor? Does the top guy in the stack lose 8 armor and the next guy down lose 1? Resoln cannot natively produce armor better than leather - is it reasonable to add a mechanic which virtually destroys the little armor they get, accessible at about the same time that they can get that armor?

I do agree that Backswing needs some work, but I would rather see axes get a chance of making a second attack (regardless of whether it hit or missed) or apply a non-stacking initiative penalty to the unit rather than a stacking armor penalty, especially since damage calculations for trained units are done on a per-figure basis.

It's a good trait idea, but not for the axe. Reducing armor would be incredibly valuable. I added once such ability to my endgame heroes and it still feels OP. Giving that to regular troops is just not possible to balance. Even at a 10% chance to sunder, you are giving too much power against armor. Ignoring armor is a better thing for trained units.

But actually the concept is quite fun. My high level swashbucklers run circles around tanks, cracking armor on each strike. It really adds some interesting mechanics to hero abilities, but it is only balanced by the fact that all other heroes have similar abilities and my units play with twice the armor of vanilla and three times the Hp.

I call it "Crack Armor."