Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nc3/3...dxc4/4.e3/4...b5/5.Nxb5/5...cxb5/6.Qf3/6...Nc6/7.Qxc6+/7...Bd7
This move blocks the check and holds everything while simultaneously developing a minor piece. a8 is guarded by the queen, b5 is guarded by the bishop, and the white queen is being attacked by the bishop. Now Qf3 is best as all Qe4 Nf6 (development for black), Qa6 e5! (the queen is out of play), Qb7 e5! (the queen is also out of play), and Qd5 e6 followed by Bb4+ (White will be forced to forfeit castling by moving his king) are all bad for white.