Last Whisper

there's no 40 AS anymore, plz correct

i dont quite understand it... or at least how it works in combination with armor pen.
f.e.: enemy got 100 armor. you got last whisper and The Brutalizer
so you penetrate 40% of the defenders armor means as if he had 60 armor. then arp of brutalizer? or first ArP then penetrate? or does it give you just 40 more penetration?
or does the higher one count?
last whisper how does it work?

40% applies after the 15.

100 armor against Last Whisper and Brutalizer.
100 - 15 ArP = 85
85 - 40% = 51

100 armor against Last Whisper.
100 - 40% = 60

Therefore, because Last Whisper is applied after,
you lose 6 ArP from Brutalizer. Alone, LW is a great
ArP item, but it shouldn't be used when stacking
ArP.

Great item on champions that don't include Brutalizer
or Black Cleaver in their build.

didn't this use to cause electricity sock type thingy?