The skills and knowledge influences you at just right moment.
You have 3 legacy points. Whenever you make an attack roll, an ability check, or a saving throw, you can spend one legacy point to roll an additional d20. You can choose to spend one of your legacy points after you roll the die, but before the outcome is determined. You choose which of the d20s is used for the attack roll, ability check, or saving throw.
You can also spend one legacy point when an attack roll is made against you. Roll a d20, and then choose whether the attack uses the attacker's roll or yours. If more than one creature spends a legacy point to influence the outcome of a roll, the points cancel each other out; no additional dice are rolled.
You regain your expended legacy points when you finish a long rest.