Koink's D&D 2024

Source: Player's Handbook

You've trained to deal particularly damaging strikes.

Once per turn, after you make an attack roll, you may subtract 2 from the result before you know whether the attack hits. If you do, you may add 1d4 to the damage roll.

Additionally, once per turn when you hit a target with an attack, you can roll the attack's damage dice twice and use either roll against the target.