It adds (or at least should add based on the description) to the Mana Mastery.
It’s diminishing returns. The higher Mastery you already have the less it will matter. If you have very low mastery it can increase Mana Surge chance by more than 10 percent points. If it’s high - you won’t notice any difference. The precise formula was on the forum some time ago, I think it hasn’t changed since:
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