Skittish is simply 80% reduction to tank player's threat generation. At base a tank does 900% increased threat. That equals to 10 times (10x) threat generated.
That means 1 damage from tank=10 threat.
Threat modifier from skittish is -80%, 0.2*10=2. So with skittish you only do 2x threat.
Basically a DPS can do up to 2x damage what tank does to a target without pulling aggro.
Source: https://www.reddit.com/r/wow/comment...ths_explained/
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