Feedback is an offensive spell cast by the Protoss High Templar. It is cast on an enemy spell caster (cannot be used on buildings), depleting the target's energy pool and dealing one point of damage for each energy lost. Feedback can only be cast on units with energy.
In competitive play, Feedback is used to snipe enemy spell casters, either weakening them by eliminating their energy or by outright killing them with the damage it can cause. Doing so will give the Protoss player an extreme advantage as they will not have to worry about hazardous spells such as EMP Round.
Feedback is not used as commonly against a Protoss opponent, as the only viable targets are the Sentry and the High Templar. Because of this, the High Templar is much more likely to use its energy for Psi Storm rather than Feedback. The Mothership is also a possible target, should the High Templar be able to get in range against the Mothership's supporting army. Feedback is also an appropriate counter to Oracle harass.
Against a Terran opponent, a High Templar will try to use Feedback to snipe the enemy's Ghosts. Because of the danger that Ghosts pose to Protoss armies, it is important to try and use Feedback on any and all Ghosts to weaken them. This is difficult to accomplish though, as an EMP will prevent the opportunity for Feedback to be used at all. Taking a watchtower outside the opponents natural can help the player target ghosts if the Terran moves out.
Feedback is a rarely used spell in PvZ matchups owing to the enormous strength of Psi Storms against Zerg unit clusters. However, it does come into play against Vipers, Infestors and Overseers. Vipers are very vulnerable as they can be killed in one shot if their energy reaches 150. Infestors have only 90HP and up to 200 energy; in addition, the most commonly-used Infestor spell, Fungal Growth, requires 75 energy. If a Zerg player has Infestors in his mix, it is a safe assumption that they have >75 energy, and thus can be severely weakened if not killed by a Feedback snipe. Overseers are much more robust but a Feedback can still knock out a large portion of their health, making them more vulnerable to Stalker snipe.