CrushChance calculates your chance to be hit with an unmitigated crushing blow. It is based on crushing blow and combat table theory, see: http://www.wowwiki.com/Crushing_blow http://www.wowwiki.com/Attack_table http://evilempireguild.org/guides/combattable.php
Crush Chance recreates your combat table and updates whenever your auras or equipment change. This combat table recreation can be seen in the FuBar tooltip. The combat table accurately accounts for effects of the level difference between you and the mob level. If you set the mob level to zero it uses the level of the last target you had. Crush Chance also detects if you have a shield equipped or not and shows a block chance of zero if not. The addon will update whenever your auras or equipment change, and also if you gain defense skill. Crush Chance will accurately account for the bonus defense from Warrior and Paladin talents as well as resilience gear.
The addon displays your chance to be crushed in a small number you can drag around the screen. Left-clicking this frame toggles the dual wield setting. If you have the addon on FuBar, the crush chance number will be shown there.