Twitch is the name for a line of necromancer spells designed to trade mana from the necromancer to another caster.
With the release of The Serpents Spine expansion and the introduction of Out of Combat enhanced regeneration, twitch's utility has become more limited in 99.9% of all situations. Twitch is now only beneficial to use as a recovery expediting feature to a cleric immediately after a rez until the Out of Combat mode is entered. Twitch can also still serve a very minor role in exceptionally long raid fights for undergeared & underAA'd clerics, however, twitch is of no use elsewhere during normal game play.
Perhaps the best usage of this old dead line is to fill the pools of low level small mana pooled Enchanter and Cleric alts so that they can perform tradeskill subcombines more quickly. With the availability of Dwerium Trio jewelry in SoF however even this is no longer really useful.