Fingers of Frost - Gives your chill effects a 7/15% chance to grant you the fingers of frost effect, which treats your next two spells as if the target were frozen. In addition your Ice Lance spell if cast during the duration will increase your spell damage by 5/10% of your spellpower and increase the critical damage bonus of your Fire spells by 25/50% for 10 seconds or one Fire spell. Lasts 15 seconds.
Basically what this does is give you a choice, either you spam more Frost(fire)bolts and just keep raking in the crits with FoF, or you use an Ice Lance if you have a Brain Freeze up, or you are gambling on getting one. Even if you don't get the Freeze proc you can lay down a FB hoping it will crit (in PvP), or just use the spell damage buff as a means to spam more powerful Frostbolts. This provides a buff to players who don't want to take Brain Freeze as well as the ones that do. I'm not much for the numbers game, but I think a 10sec buff of 10% bonus spell damage is probably more DPS than what will be lost with one Ice Lance.