Your spell criticals have a X% chance to cause your next Smite or Flash Heal spell to be instant cast, cost no mana but be incapable of a critical hit. This effect lasts 10 sec.
In this case you should note that besides increasing efficiency, Surge of Light also slightly affects effectivity of Smite by taking 1 Global cooldown to cast Smite with zero crit chance. The highest DPS gain from this talent is achieved with a critical strike chance of 29%. After this point, the effectivity bonus slowly decreases with each additional percent crit chance. At 67% crit chance, Surge of Light actually decreases Smite DPS, although this is more of a theoretical value.
Usung with Flash Heal
While note above still actual for "Surge healing", there is at least one more thing to take into consideration. You may wish to increase your mana efficiency with theese mana-free casts, but doing that requires you to make additional instant cast and thus, use extra Global cooldown. As Flash Heal is minor (that is the main difference from "Surge Smiting", where you will cast the same Smite spell, not some minor variation) healing spell and provides relatively little heal, "Surge healing" may sometimes reduce your HpS. On other hand, you will often have some kind of instant-cast "emergency spell", if you get enough critical heals to keep Surge of Light buff up for most of the time. This instant-cast heal may be used when your common Power Word: Shield is unavailable, for example, due to Weakened Soul debuff. Consider theese notes before deciding, if you want to spend your talent points in this way or not.
