Enduring
This class abilityClass AbilitySpend the required amount of tenacity to trigger a class ability. You may gain 1 light fatigue to change battle form before resolving the ability. does not require tenacityTenacityA resource tracked using the peg slots at the bottom right side of your adventurer mat. Accumulated tenacity remains until it is spent to trigger a class ability or reduced by another effect. to be spent. Instead, it is active while your tenacity is equal to or greater than the required value. The ability is no longer in effect if your tenacity drops below this number.
Do I have to use an Enduring ability?
Even if they do not use the word "may", innate abilities (from class sheets) as well as any "Enduring" class abilities, are optional. You are not forced to use them if you do not want to.
Source