Rush of Essence

Rush of Essence is a Witch Doctor passive skill, unlocked at level 36.

In-game
Spirit spells include:
 * Haunt
 * Horrify
 * Mass Confusion
 * Soul Harvest
 * Spirit Barrage
 * Spirit Walk

Note that for Haunt and Spirit Barrage, due to the cost of these spells being no higher than the amount restored, this passive effectively refunds the entire Mana cost, allowing them to be cast almost without restrictions.