Blastoneuron 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

Flying When this creature enters, exile target creature card with mana value 3 or less from your graveyard. Create a token that's a copy of it. That token gains haste. Sacrifice it at the beginning of the next end step. Whenever one or more cards leave your graveyard, each opponent loses 1 life and you gain 1 life.