Deathbringer’s Will – the trinket you’ve all probably seen but I finally checked earlier and basically we have
Equip: Increases your armor penetration rating by 155.
Equip: Your attacks have a chance to awaken the powers of the races of Northrend, temporarily transforming you and increasing your combat capabilities for 30 seconds.
Of course, that is the normal mode version, the hardmode version obviously has slightly increased stats.
What people are saying this internal cooldown is, is a +600 to your primary stat, so in a hunters’ case, +600 agility, whilst also transforming you.