Black sacrifices a pawn for rapid development and activity. Main ideas: ...Ng4 pressure vs f2/e5, or Fajarowicz 3...Ne4 systems.
Sources: Wikipedia, Chess.com opening guides, 365Chess databases.
Opening moves: 1.d4 Nf6 2.c4 e5 3.dxe5 Ng4
Update the move sequence above to match this opening.