What is it?

The Center Game (1 e4 e5 2 d4) is one of the oldest of all chess openings. Both by itself and through its cousins the Danish Gambit (2...exd4 3 c3) and the Goring Gambit (2 Nf3 Nc6 3 d4 exd4 4 c3), the opening played a major role in the "romantic" era of swashbuckling chess back in the 19th century. The Center Game's focus on immediate central tension (and the almost immediate release of that tension) has given it something of a simplistic air, and it is no longer much played at the higher levels of chess.

But, and this should be emphasized, the opening is not unsound and continues to be ventured occasionally at the "lower" levels of chess, where most of us mortals play. It is in something of a renaissance at the moment in blitz chess thanks to the efforts of players such as correspondence chess master Craig Collister. He uses the opening for its surprise value--though it is straightforward, it gives you a bit of a shock if you're not ready for it.

The Peterson Defense has a similar approach. In most of the literature on the Center Game, you'll find comments such as "Black's 2...exd4 [is] the only sound response" (Cafferty & Hooper, A Complete Defense to 1 e4, 1986). And indeed, there are not many sound options.

• Direct protection by 2...f6 is inadvisable for the kingside weakness it creates, and in fact a sequence such as 2...f6 3 dxe5 fxe5 4 Qh5+ is disastrous for black. Probably best is 3...Nc6 4 exf6 Nxf6 where black, a pawn down, at least has some developmental advantage.
  
• Direct protection by 2...d6, the Maroczy Defense, may fail against the queenswap line (3 dxe5 dxe5 4 Qxd8+) and is probably best played as a gambit (i.e., 3 dxe5 Bd7, the Philidor Gambit of the Maroczy Defense).
  
• Direct protection by 2...Nc6 is interrupted by the space-and initiative-gaining 3 d5 Nce7 (not the piece-losing 3...Nd4?).
  
• 2...d5, technically the Beyer Countergambit of the Blackmar-Diemer, just looks silly, though I suppose it's playable on some level. Note that 3 dxe4 dxe5 allows the queenswap and trouble for black.
  

I've essayed 2...Nf6 many times; and in almost every case, it stops white dead in his tracks. Not because it's brilliant, not because it's winning...but because it's a solid move that can't be directly refuted, one that is of some independent interest, one that requires some calculation. In any timed game, particularly a 3 0 or 5 0 blitz session, this factor could be key.