The priest becomes surrounded with anti-magic energy. Any successful spell cast against the priest will burn X of the attacker's Mana, causing 1 Shadow damage for each point of Mana burned. Lasts 15 sec.
Feedback is a Shadow-based short-lived buff. As long as it is active, any spell cast against you will burn enemy's mana and damage them. It is only available to Human Priests.
In the third war, Feedback was an ability used by human arcane towers and Blood ElfSpellbreakers with similar effects on mana using units.
Although Feedback is a highly situational spell (both due to the duration and cooldown), it can be extremely useful in some situations. For example, a mage's Pyroblast can be successfully countered by quickly casting Power Word: Shield and Feedback.
This spell appears to proc on every tick of a channeled spell. Because of this, using it when attacked by channeled spells is a very fast and mana efficient way to hurt an enemy caster's mana, channel time, and health.