The 20th anniversary edition of Vampire, The Masquerade says that you have to declare the use of any Disciplines at the start of your turn (P. 271), but then in some casses like Potence, it is written that you may reflexively spend a blood point to active it, and I'm not sure what that means.
About Fortitude, in Vampire, The Dark Ages 20th anniversary edition says that "Once per turn, she may spend a blood point to automatically soak her Fortitude in damage, instead of adding it to her Stamina." The Masquerade book doesn't even mention the possibility of activating Fortitude wich makes me question if they just forgot to mention it or if they intentionally removed this perk.
The ideal use of Potence would be to activate it only if you hit your target and Fortitude only when you're hit, but I'm not sure if that's how it works.
So, when do I have to/can declare to activate these Disciplines?