Smog Smasher 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

Menace Start your engines! Whenever one or more creatures you control deal combat damage to a player, conjure a duplicate of target nontoken creature into exile. Max speed — At the beginning of combat on your turn, put all cards exiled with this creature onto the battlefield. They gain haste. Sacrifice them at the beginning of the next end step.