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Relentless (passive skill): "While below 20% life, all skills cost no fury and damage taken is reduced by 50%."
Taking 14% damage while having 20% health is equivalent to taking 100% damage while having 143% health.
So to make it plain easy to understand, you have more effective health than a barbarian does normally even though you are at 20% hp.
This will hopefully clear up some confusion as to how this build works.
Here's how this works, let the enemy mobs get you down to 20% health to let relentless kick in. Then you activate ignore pain, battle rage, wrath of the berserker, call of the ancients and earthquake all at once (a quick 1-2-3-4 across your keyboard).
The 20% dodge rate of wrath of the berserker, the 65% damage reduction of ignore pain, and the 50% damage reduction of relentless will keep you alive the rest of the time.
The damage that you take will be (1-0.20)(1-0.65)(1-0.50) = (0.80)(0.35)(0.50) = 0.14, a mere 14% damage.
Long before ignore pain wears off, you will be swimming in health orbs from earthquake.
Earthquake will be your main damage dealer. It does 2000% damage over 8 seconds, which is doubled by the 100% damage increase of wrath of the berserker, a further 30% increase from battle rage, and 30% from brawler.
That's a total of (2000%)(2)(1.3)(1.3) = 6760% damage
The stacking formula is correct, active skills increase damage multiplicatively. However, I'm not sure how/if the 25% attack speed bonus of wrath of the berserker affects spells like earthquake. If it does, the total would be (2000%)(2)(1.3)(1.3)(1.25) = 8450% damage
For the mobs that are left, call of the ancients and seismic slam will finish them off.
For bosses, just switch the rune of seismic slam to stagger (70% chance of 1.5s stung). This should be enough to pull a semi-perma stun on those bosses (this might sound cheap, but blizzard designed the skill and rune thinking you would actually have to spend 30 rage for each attack).
So for comparison, a wizard does 110% with magic missile, her basic attack. For her to do 6760% damage, she would have to attack 61 times with her magic missile. And that's not accounting for the fact that earthquake does 6760% damage to every enemy within 18 yards, while magic missile only hits one target.
Now the downside to this build is that you have to wait for the 90 second cool down between mobs. The upside is that you can farm inferno mobs that other classes can't even dream of touching.