Talk:Armor penetration/@comment-4247199-20130320135158/@comment-5482772-20130423055654

http://na.leagueoflegends.com/board/showthread.php?p=32707861#32707861

"Penetration can only bring the mitigation stat to 0, Reduction can go beyond. Negative mitigation gives you a damage amp equal to the damage reduction you would have gained if you had positive mitigation with the same magnitude."

In other words:

"Damage multiplier = 2 − 100 / (100 − Armor) if Armor ≤ 0 "