Fracture is a very minor dps gain if during a certain phase of the fight the number of gcds you can land on the boss is 3n+1 before you have to disengage and do something else. For example if you have exactly 7 gcds before you have to disengage from the boss, you should do 1 goring blade combo, 1 royal authority combo, and fracture, instead of fast blade on that 7th gcd. Note that if the boss disappears most of the time your dots don't tick until it appears again, so you should use fracture only if you have to disengage but the boss is still targetable and taking damage. I'd say it's not really worth the hassle since the gain is very small. I'm not completely sure about this though, so take it with a grain of salt.