Gives you a X% chance to get an extra attack on the same target after hitting your target with your Sword.
Sword Specialization gives the Warrior a chance to deliver a second blow to the same target, while wielding a one or two-handed sword.
It is a common misconception that Sword Specialization can only proc off of normal (white) swings. This is not the case and is likely confusion over Windfury Totem. Windfury only procs off of normal (white) swings. Sword Specialization procs from all damaging attacks, including special attacks.
The extra attack is normal melee swing (white attack) and is always done by the weapon in the main hand.
It counts Whirlwind as a single attack for chance of proc.