Unchained is used when you know you're not about to get hit by big damage within the next combos to get another 5 stacks of wrath for IB; it's there to maximize dmg while in Defiance. I use 5 pre-existing wrath stacks to get IB in just before a tank crunch then infuriate > Unchained to hammer in more DPS. Great for supplementing a raid that wants to push phases.