So some stuff like Innervate gives 100% increased mana efficiency. I assumed this would just make my mana costs zero for the duration, but it didn't. Additionally, Shield Rush with 5 points into the 20% mana efficiency node (Rush Mastery) also didn't make that free.
Could someone please explain to me what mana efficiency actually does?
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If it helps conceptualise it the actual formula is
mana cost = raw mana cost / (1 + mana efficiency)
This last example would be able to translate the very way you stated mana efficiency would, I believe.
It's interesting that you bring this up, mana efficiency was originally called "increased mana cost divider", which is more descriptive. However we decided against going with that name as it was pretty long, and could be confused with "increased mana cost" at a glance, which would give entirely the wrong impression.
Stats like mana efficiency whose exact mechanics are not clear from their name will have detailed tooltips in game, and many, such as tenacity, already do. The only reason why mana efficiency doesn't yet is because at the moment you can't get it on items or passives so there's no character sheet description for it.
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