Defiance increases the threat generated by your attacks and abilities in Defensive Stance. It is a passive Protection talent, and is always active. The threat increase is multiplicative with the Defensive Stance threat modifier.
For a tanking Warrior, Defiance is a very useful ability. It increases the Warrior's base threat multiplier in Defensive Stance to 149.5%. This multiplier applies to all threat produced by the Warrior, from auto-attacks to abilities with innate threat such as Heroic Strike, Revenge, Sunder Armor, Shield Bash and Shield Slam, and the damage components of these abilities as well. It applies to Demoralizing Shout and Battle Shout as well.