Pets that deal physical damage will apply stacks, even if they don't apply on-hit effects or spell effects.
The very first instance of physical damage does not get the benefit of the armor reduction. The physical damage dealt is calculated first, and the debuff is applied after. This means the full benefit of the reduction is seen on the seventh hit, after all six stacks have been applied.
Armor reduction and penetration effects stack, allowing an attacking character to benefit simultaneously from Black Cleaver shred, penetration runes, other items such as Last Whisper, and champion abilities that circumvent armor.
In conjunction with The Black Cleaver at 6 stacks:
Trundle'sSubjugate reduces the armor of his target by 39.2% to 54.4% in the next 4 seconds (the effect does not apply any stacks). The armor reduction lasts for another 4 seconds after the drain completes.
Vi'sDenting Blows reduces the target's armor by 39.2% for 4 seconds after every third consecutive hit.
Wukong'sCrushing Blow reduces the armor of the target hit by 31.6 / 35.4 / 39.2 / 43 / 46.8% for 3 seconds.
Revised Passive: Dealing physical damage to an enemy champion Cleaves them, reducing their armor by 5% for 6 seconds, stacking up to 6 times.
The passive is now unique.
New Unique Passive:Rage: Dealing physical damage grants 20 movement speed for 2 seconds. Assists on Cleaved enemy champions or kills on any unit grant 60 movement speed for 2 seconds instead. This movement speed is halved for ranged champions.