As a reaction, which you use when you make a skill check using Strength, Dexterity, or Constitution or you make a Fortitude or Reflex saving throw, you can focus your mind to overcome your body’s limitations. You make the roll with advantage. You can do so after the roll but before any effects of the roll occur.

If the roll already has advantage or disadvantage, you cannot apply another instance of advantage or disadvantage and you must use this before the roll to cancel out any advantage or disadvantage.