For example, Rammus Taunt can make you do additional physical damage by reducing the enemys Armor to a negative amount. But the spell clearly says that it REDUCES Armor...
Does it work with Flat Penetration too?
Lets say you have 45 flat Magic Penetration, the enemy champ has no bonus MR so 30 MR, will you just do true damage or do the remaining 15 count towards a -15 magic Resistance, giving you bonus damage?
EndlessSorcerer3 votesPenetration won't reduce resistences below 0.
Reduction can.
JustMyBassCannon1 votesNo, penetration does not go below 0
Stiv12341 votesNo, that only works with flat MR/AR reduction, like 
. Penetration can only go as far as 0 MR/AR, making you deal true damage.
Fiveofswords1 votesIts very unlikely that people really stay around 30 mr. If they do, they are going to be extremely easy for you to burst down even with wasted penetration. Most people will at least get some mr from runes or something....and if you start doing decent damage, they will be forced to get some mr or simply continue being killed over and over by you. SO you shouldnt worry about the 'wasted' 15 penetration.
like basically if you have 45 mpen, and nobody on the otehr team is getting more than 30 mr, then jsut get a mejas. cause you are goign to rack up some kills.
these considerations do however alter how useful voidstaff is. if you are using ability on someone with like 60-80 mr (which is kinda normal for late game adc then 45 penetration plus voidstaff is a negligable difference. the voidstaff is not really worth its gold. However, if you have zero penetration and used that money/runes on ap then voidstaff is much more of a power spike. The better choice tends to be a question of how high your ap ratios are on your abilities. flat pen is good for people with high base damage and somewhat low ap ratio (brand, karthus for example) while voidstaff winds up being the better path for people with high ap ratio (fizz, annie for example)
67chrome1 votesCan flat penetration make you do bonus damage?
Nope.
Flat Reductions can though.
But flat reductions are applied prior to flat penetrations, necessitating your flat reductions alone exceeding your opponents armor or MR (whichever you're targeting).
The Sword Saint1 votesOnly reduction can bring the effective armor for damage calculation below 0, and yes in that case your damage will be bigger than true damage.
