Reduces the casting time of your Mana Burn spell by X sec.
Mana Burn is a priest discipline talent reducing the cast time for Mana Burn this is sought after for a disc pvp build.
Cast time reduction
Really depends what you're going to be fighting. Paladins, Shamans, Hunters and other Priests hate this spell, and going from 3 seconds to 2 seconds helps a lot with this. Also, this damage is considered shadow damage, so it will heal you with Vampiric Embrace, even though it doesn't get powered up by any talents and does not scale to gear (+damage). Depends on play style.