In the Fallout series, critical hits have always been a major thing. It's hard to show that in the homebrew dice system we're using, but we want to keep some of that Fallout flavor, so here's how it works. Any time you roll a 10, you may re-roll that 10 to potentially add more successes to your pool. If the re-roll is a 10, you can re-roll again, until you stop getting 10s. These re-rolls are always done at the same difficulty rating as the original roll.