When you roll a Dance Die, you can flip the die over and use the number on the bottom. Note that for a balanced die, the top and bottom numbers add up to one more than the die's largest number. Once you use this feature, you can't use it again until the start of your next turn.
You can use this feature a number of times equal to your Dexterity modifier (minimum of once). You regain one expended use when you finish a Short Rest, and you regain all expended uses when you finish a Long Rest.