This addon accelerates key bindings so that they are activated by key press rather than key release. This allows you to activate your abilities faster than you could otherwise. This can really make a difference for situations where you need to react quickly, like when casting heals, when dispelling, when interrupting spells, or in PvP. In these situations, the addon can have an effect similar to reducing your network latency by 100ms. Of course, the exact impact depends entirely upon how much time you personally spend between key press and release.
Expansion: Wrath of the Lich King
Game Version: 3.3.5a