I am working through the book mentioned and in game 8 ([Przepiorka vs Prokes](http://www.chessgames.com/perl/chessgame?gid=1242926)) at move 4 ...P-B4, which is black moves pawn to C5. The book states the following: "This move is almost indispensable in Queen Pawn openings. It is important not to play ...Kt-C6 first, as the bishop pawn must not be obstructed." Could someone shed some light on the importance of ensuring the queen's bishop pawn is unobstructed?