Crushing blow

A crushing blow is a normal hit or critical hit by an NPC that deals 150% of the damage it would otherwise cause. A player cannot perform a crushing blow (unless under Mind Control, in which case they are treated as mobs).

Chance of a crushing blow occurring
When the attacking mob is more than 4 levels above the target, there is a chance of being struck by a crushing blow. If the level difference is only 3, which is common for players being attacked by "boss" mobs (who are always treated as 3 levels higher than their attackers), then the player cannot be hit by a crushing blow.

Since patch 3.0, the crushing blows mechanics have been changed. Now only mobs whose level is 4 or more above your level can deal Crushing Blows. However, this mechanic is based on a now-hidden quantity called the target's Defense Skill, which can be reduced by a few rare debuffs such as Unbalancing Strike. For every 5 points the target's Defense is reduced, it is considered 1 level lower for purposes of receiving Crushing Blows.

The chance of being hit by a crushing blow is:
 * ((Mob's Level * 5) - (Player's Level * 5) [mininum difference 20]) * 2% - 15%

This means a mob 4 levels higher than you will have a 25% Crushing Blow chance, and a mob 5 levels higher than you will have a 35% Crushing Blow chance.