The Mystery Raceway is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Start your engines! : Add . Max Speed — , : Mill seven cards. Choose a playtest card from among them. You may cast it without paying its mana cost. If Team Cloudspire won the race, copy it and you may choose new targets for the copy. If Team Speed Demons won the race, each opponent loses life equal to its mana value. The Mystery Raceway can be your commander.