This talent has great value for both Arcane and Frost in PvP. Fine-tuned Frost PvP builds sink 20 points in Arcane to pick up this, Improved Counterspell and Magic Absorption. It greatly increases the burst provided by Arcane Barrage and Frostbolt, the builds' main spells.
/Patch 4.0.1 (12-Oct-2010): This talent is now a Tier 2 talent (requires 5 points), down from Tier 4 (required 15 points). Now only affects only Arcane spells. Your Arcane damage spells deal 2/4/6% more damage to snared or slowed targets.
Patch 3.3.3 (2010-03-23): This talent now also applies to Pyroblast damage. Your Frostbolt, Fireball, Frostfire Bolt, Pyroblast, Arcane Missiles, Arcane Blast, and Arcane Barrage spells deal 4/8/12% more damage to snared or slowed targets.
Patch 3.0.8 (2009-01-20): Now works with Arcane Blast and does bonus damage against targets afflicted with any type of slow (such as the combat slow from Thunder Clap).
Patch 3.0.3 (04-Nov-2008): Now works correctly when the Mage also knows the Molten Fury Talent.