"When the attacking mob has 15 or more points of weapon skill above the player's base defense there is a chance of being struck by a crushing blow. The chance is 2% per point of difference minus 15%. If the rating difference is exactly 15 points, which is common for players being attacked by "boss" mobs (who are always treated as 3 levels higher than their attackers), then the crushing blow chance is 15% (which is 2% * 15 - 15%).
The maximum defense that can be applied by a player against reducing crushing blows versus mobs with higher weapon skill is 5 * the player's level. A level 70 player therefore cannot apply more than 350 points of defense against crushing blows versus level 71 and higher mobs, even if the player's normal defense is higher than 350. "
"If your Defense skill is fully trained for your level, neither more Defense, nor Resilience, nor any other kind of bonus can reduce the nominal chance for you to receive a Crushing Blow. The only way to reduce your chance of receiving a Crushing Blow is for your combined chance to be missed, to dodge, to parry, and to block to be so high that the Crushing Blow chance is pushed off the bottom end of your attacker's Attack Table."
- Wowwiki
http://www.wowwiki.com/Crushing_blow_________________
Theorycraft is the last thing that is gonna help you kill a boss. Yes i will get 20 dps more or tank will get 20 TPS more, but that doesn't kill bosses, playing smart and good while being ready to improvise is what does the job.