Every time you deal a standard strike in combat, you have a chance of dealing a Critical Hit.
Criticals deal much more damage then usual (200%? Might deal damage without taking into account target's resists?).
In fact according to the relevant Stratics article the Anatomy damage bonus only applies if you "pass a check" (deal a crit?), but our Damage Calculations page reckons it always applies. I have no idea which is the truth.