The dashes would be placed as follows: Yesterday's basketball victory -the crowd was ecstatic- gurantees us a chance at the state title. This would be an em dash. Em dashes are the most versatile punctuation mark: They can replace commas, parentheses or colons.
In this case, the dashes have been placed instead of a pair of parentheses. They are less formal than the pair of parentheses but they catch more attention, they are more intrusive and therefore can be used when you want the reader to pay particular attention to the information they include.
