Wild Cards, which includes all player characters, are more competent and get to roll their Wild Die when they make trait rolls and take the higher of the two results. For instance if your Spirit is only d4, you get to roll the d4 AND d6, taking whichever result is higher. So if you rolled a 2 on the d4 and a 5 on the d6, you get to take the 5, even though that's higher than you could have possibly rolled without it!