Codex text
Sustained abilities reserve mana or stamina when they're activated, and remain on until deactivated or the character runs out of mana or stamina. Most sustained abilities include a fatigue penalty, increasing the cost to activate a subsequent spell or talent, which makes it difficult to maintain several sustained abilities at one. To manually deactivate a sustained ability, simply left-click it again in the quickbar. Note that some of these abilities, such as Defensive Fire and Rapid Shot, cannot be used simultaneously.