Gives you a X% chance of entering a Clearcasting state after any damage spell hits a target. The Clearcasting state reduces the mana cost of your next damage spell by 100%.
Arcane Concentration is a Mage talent that gives you a chance to enter a Clearcasting state after any damage spell hits a target. The Clearcasting state lasts 10 seconds, and the next damage spell will cost no mana.
This talent is very popular with all types of mages, as it is nearly a 10% reduction in mana usage over time. Reduced mana use means a mage can focus his gear on other stats, such as spell damage, spell crit or stamina, depending on the situation. In practice, the Clearcasting state can proc just as an enemy dies, meaning the buff will expire before another spell is cast. This makes the talent most useful for longer fights, such as boss encounters, but it still improves efficiency in most all situations.
One technique is to use low cast time or high mana efficiency spells such as Scorch until a Clearcasting procs. Then use a higher damage spell that has lower efficiency, such as Arcane Missiles when the casting cost is free. In instances or solo'ing, you can also cast Scorch until Clearcasting procs, then fire off a Blizzard. Combined with Arcane Potency, which at max rank gives a +30% crit chance when Clearcasting or Presence of Mind activates, this can be a lethal attack. Further, Blizzards often proc clearcasting when allowed to run to their full duration, due to the large number of times Blizzard strikes and thus has a chance at proc'ing, which can sometimes mean that you can chain two, three, or even four Blizzards for no mana cost that all (combined with Arcane Potency) have a +30% crit chance. It's a fatal barrage.
Unlike "on spell hit" effects, Arcane Concentration cannot proc off each hit of an Arcane Missiles' bolt, but only off the initial hit of AM itself.