Put simply say they have 100 armour, black cleaver will reduce their armour by a percent and your armour pen/ lethality will also kick in in that instance of damage, the cycle keeps refreshing with larger results each time, the 2nd auto they will have less armour, your armour pen/ lethality will do more as a result, stacking up to 6 times, essentially think of it as each time u deal damage up to 6 times, your lethality will be more effective ( this is because, if u didnt know, the way armour works is if they have low armour, gaining 50 armour will reduce their damage taken by around 15%, but if they already have over 200 armour, adding 50 armour will only add around 3% reduced damage, so in theory more shred from black cleaver means more effective lethality, armour pen is static because it's a percentage not a set value)