Benko gambit half accepted
1. d4 Nf6 2. c4 c5 3. d5 b5 4. cxb5 a6
White plays the queen's pawn to d4. Black develops the knight to f6. White moves the c-pawn to c4.
Black pushes the c-pawn to c5. White plays the queen's pawn to d5. Black moves the b-pawn to b5.
White captures on b5 with the c-pawn. Black pushes the a-pawn to a6.