Increases the duration of your Chill effects by X secs, reduces the target's speed by an additional Y%, and reduces the target's healing received by Z%.
Permafrost is a mage talent that increases the effectiveness of the mage's chill effects, increasing the duration by 1 sec. per rank, up to 3 sec. max, reduces movement speed by an additional amount, up to 10% at max rank 3, and reduces the target's healing received up to 20% at max rank 3. The additional reduction to movement speed is an additive amount, meaning that a snare that normally slows by 50% will now slow by 60%. This talent is popular with frost mages that do a lot of kiting, either against single mobs or when doing frost AoE grinding on multiple mobs at once.
Some spells gain a particular advantage from the increased chill duration. Blizzard, with points in Improved Blizzard normally has a chill of 1.5 seconds, which is increased to 4.5 seconds with Permafrost, three times the duration. Cone of Cold normally has a chill duration of 8 seconds, which is increased to 11 seconds with Permafrost, making the duration longer than the spell's cooldown of 10 seconds.