From Diablo Wiki
Articles related to the Monk's Mantras. These are Aura-like effects that provide long lasting passive buffs to the Monk, and often extend their bonuses to party members as well.
All Mantras have a short term (usually 3 seconds) extra bonus (usually double) and many Monks cast and recast their Mantras as often as their Spirit will permit, to get more use out of this short term buff. It's best used to time the effect with other skills (such as damage buffs) to provide short term exponential damage increases, useful to burn down larger enemies such as Elites.
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