Increases the critical strike chance of your next damaging spell by 15n% after gaining Clearcasting or Presence of Mind.
Arcane Potency is a Mage talent that increases spell critical strike chance while Clearcasting by 15% per rank, up to 30% at max rank. Clearcasting comes from the talent Arcane Concentration and Presence of Mind is a talented spell in the Arcane tree, prerequisite for obtaining Arcane Potency. With 2 points in this talent, and a 10% chance to gain Clearcasting with Arcane Concentration, the effective critical strike increase is 3%, provided that only spells capable of critical strikes are cast while Clearcasting.
This talent has somewhat peculiar synergy with Arcane Missiles, and a mage who uses Arcane Missiles as part of his rotation can get the +crit benefit twice per Clearcast from this talent; once for a cast of Missiles that procs it, and once again for a second spell that consumes the Clearcast.
Its relationship with Presence of Mind is mainly useful for PvP purposes, as it greatly increases the spell's already large burst potential by making its instant-cast spell a near-guaranteed critical strikes.
This spell applies a buff on you that is consumed by the next crittable spell that you cast, not by the spell affected by PoM or Clearcasting, though they are often the same.
This means you can cast a PoM Polymorph and benefit from the crit bonus for an actual damage spell, for instance. It also prevents abuse by players who might cast PoM and use spells cabable of critical strikes but that do not consume PoM such as Arcane Missiles. In this case, Arcane Missiles consumes the Arcane Potency buff but not PoM.