Originally Posted by
spookyy
^This
It's not that hard to understand. Every single harmful action against Nazgrim during Defensive Stance, no matter if it's a white hit, a fireball, or even a non damaging spell like curse of the elements, will trigger his rage generation. This effect hower has an internal CD to prevent enormous amounts of rage gained from one specific class (say a fast hitting melee didn't notice boss is in def stance).
Effects that have been applied and have another effect being triggered from the initial application do not trigger rage every time their effect triggers (easiest example: DoTs. Rage is gained upon applying them, but not from every single DoT ticks).
Pets, guardians, etc. are also an exception and do not trigger his rage gain during def stance.
This explanation should cover pretty much every possible situation.