If a damage buff or debuff (e.g. exhaust) is applied to a champion while they are channeling a spell (after they started the channeling) like ww' ult, kata's ult or fiddle's ult is the damage for that spell adjusted mid-channeling, or the buff/debuff has to be applied before they started channeling in order to take effect for that spell damage. What about spells that involve traveling projectiles like ez, ashe, draven, ziggs ulties? Or delayed spells like leona's ult?
I ask this as I mainly play support and when I see one of those ults, if I can't cc to stop them, I apply exhaust while they are channeling to lower their ult's damage for the remaining of the channel and I was just wondering if that works or I am just wasting my summoner's spell.
Also, as a side-note, how do you even test for something like this, or where do you get an (semi-)official answer?
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Best Answer
So I tried this stuff out a bit and I came to the following conclusion:
Every damage modification is applied Instantly
This means that the damage of a spell/dot/projectile is applied for the exact duration of the damage buff or debuff. The damage of an Ashe arrow for example is reduced only if she is exhausted on its impact. Dots work similar (I tried this with teemo shrooms): When someone walks into your shroom and you buy an item with ability power during the DOT time the dot will become stronger as soon as you have the item in your inventory (Same goes for leveling up a skill. I had this with malzahar e before). The dot damage is lowered for the duration of the exhaust on teemo and is also applied immediately. This also means that if an ezreal fires an ult for example it won't help if you exhaust him while he's channeling if the ult hits 4 seconds later.
TL;DR
Exhaust reduces damage while it's on the enemy. Not before and not after.