cr:6.4.10g

If an effect decreases the amount a prevention effect would prevent, the total prevention is determined, the prevention effect’s prevention amount is reduced, and then the prevention effect is applied to the event with the decrease.

Example: Vambrace of Determination has the text “The next prevention effect that prevents {p} damage this turn, prevents 1 less of that damage.” If Feign Death’s effect would be applied to an event of 3{p} damage, first the total prevention is established as 3, then it is reduced by Vambrace of Determination’s effect, preventing only 2 damage.

Example: Vambrace of Determination has the text “The next prevention effect that prevents {p} damage this turn, prevents 1 less of that damage.” If Bone Head Barrier’s effect has a prevention amount of 3 and would be applied to an event of 3{p} damage, first the total prevention is established as 3, the Bone Head Barrier’s prevention amount is subtracted, then the total prevention is reduced by Vambrace of Determination’s effect. Bone Head Barrier can’t prevent any more damage as its prevention amount is 0, but it only prevents 2 damage.